feat(stdlib/E6a): per-decl nominal identity for enum + union decls

Give top-level ENUM and UNION decls per-decl nominal identity so two
same-name flat enums/unions intern DISTINCT nominal TypeIds instead of
collapsing to one global last-wins entry. Establishes the reusable
non-struct register path the later E6 kind-steps (E6b error-set, E6c
protocol, E6d foreign-class) extend.

Registration side (was: stateless `type_bridge.resolveInlineEnum/Union`
`findByName` last-wins short-circuit, no Lowering access):
- Split the type_bridge inline builders into a body-BUILDER
  (`buildEnumInfo` / `buildUnionInfo`) + the existing thin interner
  wrappers (field-type positions keep the legacy single-slot path).
- Add `Lowering.registerEnumDecl` / `registerUnionDecl` mirroring
  `registerStructDecl`: build the TypeInfo, intern via
  `internNamedTypeDecl(decl_key, name_id, info, nominal_id)` under the
  per-decl nominal identity (reserved slot id, else `shadowNominalId`).
- Reroute all six enum/union registration dispatch sites (scanDecls
  const-wrapped + top-level, lowerDecls/comptime, block-local, local
  const) to the new path.

Shared infra generalized ONCE:
- Pass-0b genuine-shadow pre-pass now reserves struct/enum/union shadow
  slots of the MATCHING kind, grouped by (kind, name), via a kind-generic
  `topLevelTypeDecl` / `reserveShadowSlot`. A forward/self/mutual ref to a
  shadow name binds to the reserved nominal TypeId.
- `namedRefTid` consults `type_decl_tids` for `.enum_decl`/`.union_decl`
  before the global `findByName`.

No new per-kind resolution path: selectNominalLeaf / headTypeGate /
flatTypeAuthorCount already gate every kind. Single-author /
phantom-double-spelling names keep nominal_id 0 (byte-identical corpus).

Regressions 0795-0798 (enum + union: ambiguity over every bare-type form,
and own-wins with distinct nominal TypeIds), fail-before/pass-after:
0795/0797 exit 0 -> exit 1 with the loud "type is ambiguous" diagnostic;
0796 silently printed `own=.east` -> correct `own=.north`; 0798 hard
`field 'm' not found` error -> correct `own=5 dep=9`.

Gate: zig build && zig build test (423/423) && run_examples.sh (537/537)
all exit 0; m3te ios-sim build via the main binary exit 0.

examples/0795-modules-same-name-enum-ambiguous.sx

@@ -0,0 +1,37 @@
+// E6a — per-decl nominal identity for ENUM decls. A bare ENUM reference is
+// non-transitive AND ambiguity-checked at every site, exactly like the struct
+// leaf (0755) and the non-leaf struct forms (0767). `main` flat-imports two
+// modules that each author a same-name `Dir` enum and authors none itself, so
+// EACH of the following bare ENUM forms is a genuine collision the source cannot
+// disambiguate — and each must emit the LOUD "type 'Dir' is ambiguous" diagnostic
+// and poison the result, NEVER silently pick a global `findByName` last-wins
+// author:
+//
+//   - reflection / type-arg slot    `size_of(Dir)`
+//   - typed enum-value annotation    `d : Dir = .north`
+//   - type-as-value                  `t : Type = Dir`
+//   - type-category match arm        `case Dir:`
+//
+// Fail-before (pre-E6a): the stateless `type_bridge.resolveInlineEnum`
+// `findByName` short-circuit interned ONE global last-wins `Dir`, so every bare
+// form silently resolved to it and the program exited 0.
+
+#import "modules/std.sx";
+#import "0795-modules-same-name-enum-ambiguous/a.sx";
+#import "0795-modules-same-name-enum-ambiguous/b.sx";
+
+describe :: ($T: Type) -> s32 {
+    r := if T == {
+        case Dir: 1;
+        else: 0;
+    }
+    r
+}
+
+main :: () -> s32 {
+    sz := size_of(Dir);
+    d : Dir = .north;
+    t : Type = Dir;
+    k := describe(s64);
+    0
+}

examples/0795-modules-same-name-enum-ambiguous/a.sx

@@ -0,0 +1,4 @@
+// One of two flat-imported authors of a same-name `Dir` enum. With both modules
+// flat-visible from a file that authors none itself, every bare reference to the
+// name is genuinely ambiguous.
+Dir :: enum { north; south; east; west; }

examples/0795-modules-same-name-enum-ambiguous/b.sx

@@ -0,0 +1,4 @@
+// The second flat-imported author of a same-name `Dir` enum. A separate nominal
+// identity from a.sx's `Dir`, so each bare reference is a real collision the
+// importing source cannot disambiguate.
+Dir :: enum { north; south; east; west; }

examples/0796-modules-same-name-enum-own-wins.sx

@@ -0,0 +1,22 @@
+// E6a — own-wins-over-flat for ENUM per-decl nominal identity. `main` flat-imports
+// `dep.sx` (which authors `Dir { east; west }`) AND authors its OWN `Dir { north;
+// south }`. A bare `Dir` reference in `main` resolves to `main`'s OWN author, not
+// the flat-imported one (the querying source's author wins outright — no
+// ambiguity), so `d : Dir = .north` binds `main`'s enum (whose `.north` variant
+// dep's `Dir` lacks) while `dep_dir()` returns dep's DISTINCT `Dir`.
+//
+// Fail-before (pre-E6a): the stateless `type_bridge.resolveInlineEnum` `findByName`
+// short-circuit interned ONE global last-wins `Dir`, so `main`'s `Dir` and dep's
+// `Dir` collapsed to a single nominal — `.north` would resolve against whichever
+// author won the global slot, silently wrong with no diagnostic.
+
+#import "modules/std.sx";
+#import "0796-modules-same-name-enum-own-wins/dep.sx";
+
+Dir :: enum { north; south; }
+
+main :: () -> s32 {
+    d : Dir = .north;
+    print("own={} dep={}\n", d, dep_dir());
+    0
+}

examples/0796-modules-same-name-enum-own-wins/dep.sx

@@ -0,0 +1,6 @@
+// A flat-imported module authors its OWN `Dir { east; west }`. The importing file
+// (`main`) ALSO authors a `Dir` — its own author must win there (own-wins), while
+// this module's `Dir` stays a DISTINCT nominal type used by `dep_dir`. The variant
+// sets are disjoint, so a cross-binding to the wrong `Dir` is a hard compile error.
+Dir :: enum { east; west; }
+dep_dir :: () -> Dir { return .west; }

examples/0797-modules-same-name-union-ambiguous.sx

@@ -0,0 +1,36 @@
+// E6a — per-decl nominal identity for UNION decls. A bare UNION reference is
+// non-transitive AND ambiguity-checked at every site, exactly like the enum
+// (0795) and struct (0767) forms. `main` flat-imports two modules that each author
+// a same-name `Pair` union and authors none itself, so EACH of the following bare
+// UNION forms is a genuine collision the source cannot disambiguate — and each must
+// emit the LOUD "type 'Pair' is ambiguous" diagnostic and poison the result, NEVER
+// silently pick a global `findByName` last-wins author:
+//
+//   - reflection / type-arg slot    `size_of(Pair)`
+//   - typed annotation               `u : Pair = ---`
+//   - type-as-value                  `t : Type = Pair`
+//   - type-category match arm        `case Pair:`
+//
+// Fail-before (pre-E6a): the stateless `type_bridge.resolveInlineUnion`
+// `findByName` short-circuit interned ONE global last-wins `Pair`, so every bare
+// form silently resolved to it and the program exited 0.
+
+#import "modules/std.sx";
+#import "0797-modules-same-name-union-ambiguous/a.sx";
+#import "0797-modules-same-name-union-ambiguous/b.sx";
+
+describe :: ($T: Type) -> s32 {
+    r := if T == {
+        case Pair: 1;
+        else: 0;
+    }
+    r
+}
+
+main :: () -> s32 {
+    sz := size_of(Pair);
+    u : Pair = ---;
+    t : Type = Pair;
+    k := describe(s64);
+    0
+}

examples/0797-modules-same-name-union-ambiguous/a.sx

@@ -0,0 +1,4 @@
+// One of two flat-imported authors of a same-name `Pair` union. With both modules
+// flat-visible from a file that authors none itself, every bare reference to the
+// name is genuinely ambiguous.
+Pair :: union { f: f32; i: s32; }

examples/0797-modules-same-name-union-ambiguous/b.sx

@@ -0,0 +1,4 @@
+// The second flat-imported author of a same-name `Pair` union. A separate nominal
+// identity from a.sx's `Pair`, so each bare reference is a real collision the
+// importing source cannot disambiguate.
+Pair :: union { f: f32; i: s32; }

examples/0798-modules-same-name-union-own-wins.sx

@@ -0,0 +1,23 @@
+// E6a — own-wins-over-flat for UNION per-decl nominal identity. `main` flat-imports
+// `dep.sx` (which authors `Pair { a }`) AND authors its OWN `Pair { m }`. A bare
+// `Pair` reference in `main` resolves to `main`'s OWN author, not the flat-imported
+// one (the querying source's author wins outright — no ambiguity), so `p : Pair`
+// here binds `main`'s union (whose `m` field dep's `Pair` lacks) while `dep_pair()`
+// uses dep's DISTINCT `Pair`.
+//
+// Fail-before (pre-E6a): the stateless `type_bridge.resolveInlineUnion` `findByName`
+// short-circuit interned ONE global last-wins `Pair`, so `main`'s `Pair` and dep's
+// `Pair` collapsed to a single nominal — `p.m` would resolve against whichever
+// author won the global slot, silently wrong with no diagnostic.
+
+#import "modules/std.sx";
+#import "0798-modules-same-name-union-own-wins/dep.sx";
+
+Pair :: union { m: s32; }
+
+main :: () -> s32 {
+    p : Pair = ---;
+    p.m = 5;
+    print("own={} dep={}\n", p.m, dep_pair());
+    0
+}

examples/0798-modules-same-name-union-own-wins/dep.sx

@@ -0,0 +1,10 @@
+// A flat-imported module authors its OWN `Pair { a }`. The importing file (`main`)
+// ALSO authors a `Pair` — its own author must win there (own-wins), while this
+// module's `Pair` stays a DISTINCT nominal type used by `dep_pair`. The field sets
+// are disjoint, so a cross-binding to the wrong `Pair` is a hard compile error.
+Pair :: union { a: s32; }
+dep_pair :: () -> s32 {
+    p : Pair = ---;
+    p.a = 9;
+    return p.a;
+}

examples/expected/0795-modules-same-name-enum-ambiguous.exit

@@ -0,0 +1 @@
+1

examples/expected/0795-modules-same-name-enum-ambiguous.stderr

@@ -0,0 +1,23 @@
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:32:19
+   |
+32 |     sz := size_of(Dir);
+   |                   ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:33:9
+   |
+33 |     d : Dir = .north;
+   |         ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:34:16
+   |
+34 |     t : Type = Dir;
+   |                ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:25:14
+   |
+25 |         case Dir: 1;
+   |              ^^^

examples/expected/0795-modules-same-name-enum-ambiguous.stdout

@@ -0,0 +1 @@
+

examples/expected/0796-modules-same-name-enum-own-wins.exit

@@ -0,0 +1 @@
+0

examples/expected/0796-modules-same-name-enum-own-wins.stderr

@@ -0,0 +1 @@
+

examples/expected/0796-modules-same-name-enum-own-wins.stdout

@@ -0,0 +1 @@
+own=.north dep=.west

examples/expected/0797-modules-same-name-union-ambiguous.exit

@@ -0,0 +1 @@
+1

examples/expected/0797-modules-same-name-union-ambiguous.stderr

@@ -0,0 +1,23 @@
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:31:19
+   |
+31 |     sz := size_of(Pair);
+   |                   ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:32:9
+   |
+32 |     u : Pair = ---;
+   |         ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:33:16
+   |
+33 |     t : Type = Pair;
+   |                ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:24:14
+   |
+24 |         case Pair: 1;
+   |              ^^^^

examples/expected/0797-modules-same-name-union-ambiguous.stdout

@@ -0,0 +1 @@
+

examples/expected/0798-modules-same-name-union-own-wins.exit

@@ -0,0 +1 @@
+0

examples/expected/0798-modules-same-name-union-own-wins.stderr

@@ -0,0 +1 @@
+

examples/expected/0798-modules-same-name-union-own-wins.stdout

@@ -0,0 +1 @@
+own=5 dep=9