fix(lower): genuinely-undeclared type → diagnostic + .unresolved (no silent stub) [stdlib E3]

Phase E3: remove the silent empty-struct fall-throughs in type resolution for
genuinely-undeclared names, replacing them with a real "unknown type" diagnostic
+ the dedicated `.unresolved` sentinel (already present, with the sizeOf @panic
tripwire) — the REJECTED-PATTERN this project bans.

Split `TypeHeadResolution.undeclared` into `.forward` (a real author not interned
yet — self/forward/mutual/foreign reference, adopted on registration via
internNamedTypeDecl) vs `.undeclared` (NO author anywhere). `resolveNominalLeaf`:
- `.pending` / `.forward` keep the empty-struct stub the type adopts on register.
- `.undeclared` in a NON-main (imported/library) module — which the
  UnknownTypeChecker trusts and never walks — emits "unknown type 'X'" + poisons
  with `.unresolved`. In the MAIN file the checker owns the diagnostic (and a
  valid unbound generic leaf legitimately lands here), so the leaf keeps the
  legacy stub and does not double-report.

Also convert the `parameterized_type_expr` constructor-head fallback
(resolveParameterizedWithBindings): an unresolvable base now emits + returns
`.unresolved` (mirroring the `.call`-node sibling) instead of a 0-field stub
that mis-sizes `b.field` / `b.len`. Threads the reference span through both
callers.

Triage of the other empty-struct sites (all load-bearing on the green suite or
unable to distinguish forward from undeclared — KEPT): resolveNamed's legacy
namer (forward/generic/Self/foreign-opaque: R/Self/Object/Array), the
foreign-class struct + JNI Self placeholders, the shadow-slot reservation, the
type_bridge stateless pack/generic namer, and the struct-literal inference
fallback (front-run by the leaf; 0 suite hits).

Regression: examples/0759-modules-undeclared-type-in-import — an undeclared type
in an imported module now errors (exit 1) instead of silently compiling (the
pre-fix code printed `thing.x = 42`, exit 0).

Gate: zig build; zig build test (423/423 + LSP corpus sweep); run_examples 497
passed / 0 failed (prior 496 byte-identical); m3te ios-sim build exit 0.

examples/0759-modules-undeclared-type-in-import.sx

@@ -0,0 +1,22 @@
+// A genuinely-undeclared type name used in an IMPORTED (non-main) module must
+// emit a clean "unknown type" diagnostic, not silently compile.
+//
+// The `UnknownTypeChecker` only walks MAIN-file decls — imported / library
+// modules are trusted and never checked. So an undeclared type name in an
+// imported module used to fall through the type leaf's empty-struct stub and
+// silently fabricate a 0-field struct: `make_thing()` below compiled and ran
+// (printing `thing.x = 42`) even though `lib.sx` references the non-existent
+// type `Coordnate`. The source-aware nominal leaf now poisons a genuinely-
+// undeclared name with the `.unresolved` sentinel and emits the diagnostic at
+// the reference, so the typo surfaces instead of mis-sizing `Thing` downstream.
+//
+// Expected: `error: unknown type 'Coordnate'` pointing into lib.sx; exit 1.
+// Regression (stdlib E3).
+#import "modules/std.sx";
+
+#import "0759-modules-undeclared-type-in-import/lib.sx";
+
+main :: () -> s32 {
+    print("thing.x = {}\n", make_thing());
+    return 0;
+}

examples/0759-modules-undeclared-type-in-import/lib.sx

@@ -0,0 +1,14 @@
+// Flat-imported helper. `Coordnate` is a typo — no such type is declared
+// anywhere. Because this module is imported (not the main file), the
+// `UnknownTypeChecker` trusts it and never walks it, so the type leaf is the
+// sole guard against the silently-fabricated empty-struct stub.
+Thing :: struct {
+    x: s32;
+    y: Coordnate;
+}
+
+make_thing :: () -> s32 {
+    t : Thing = ---;
+    t.x = 42;
+    return t.x;
+}

examples/expected/0759-modules-undeclared-type-in-import.exit

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

examples/expected/0759-modules-undeclared-type-in-import.stderr

@@ -0,0 +1,5 @@
+error: unknown type 'Coordnate'
+  --> examples/0759-modules-undeclared-type-in-import/lib.sx:7:8
+   |
+ 7 |     y: Coordnate;
+   |        ^^^^^^^^^

examples/expected/0759-modules-undeclared-type-in-import.stdout

@@ -0,0 +1 @@
+