feat(lang): universal backtick raw identifier — valid in value, decl, AND type position [F0.6]

AGRA ruling (attempt 4): `` `name `` is THE LITERAL identifier `name`, usable in
EVERY position — the backtick only means "treat this token as a plain identifier,
never the reserved keyword/type", and is never part of the name's text.

- Raw in TYPE position is now VALID (reverses attempt-2 "raw is not a type"):
  `parseTypeExpr` emits a raw `type_expr`; `TypeResolver.resolveNamed` gains a
  `skip_builtin` flag (threaded from `te.is_raw` via lower.zig + type_bridge) so a
  `` `s2 `` reference resolves to a `` `s2 ``-declared type (struct/enum/union/alias),
  else a normal "unknown type 's2'" error (reportIfUnknownType skips the builtin
  exemption when raw). Bare `s2` in type position stays the builtin int.
- Every declaration-name site is is_raw-exemptible: `is_raw` added to TypeExpr +
  StructDecl/EnumDecl/UnionDecl/ErrorSetDecl/ProtocolDecl/ForeignClassDecl/UfcsAlias/
  NamespaceDecl/ImportDecl/CImportDecl/LibraryDecl; parser threads name_is_raw to
  every decl parse fn; namespace imports carry it through imports.addNamespace.
  Typed-const path (`` `s2 : s64 : 5 ``) now threads name_span+is_raw (fixes the
  1:1-caret bug).
- Check<->exemption made structurally symmetric: checkBindingName/checkDeclName take
  is_raw as a REQUIRED argument and skip inside the check, so no call site can
  validate a name without honoring the exemption (the desync cause of prior rounds).
- Bare reserved-name declarations of every kind still error (0076 preserved);
  `#import c` foreign names stay auto-raw + bare-callable.

specs.md + readme.md updated to the universal model. issue 0089 RESOLVED banner
rewritten. Examples: replace 1139 (raw-not-a-type) with 0154 (raw type reference);
add 0155 (typed const + union tag) and 1141 (bare type-decl negatives).
Gate: zig build + zig build test + run_examples (426 passed, 0 failed).

examples/expected/0154-types-backtick-raw-type-reference.exit

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

examples/expected/0154-types-backtick-raw-type-reference.stdout

@@ -0,0 +1,5 @@
+struct = 7
+alias  = 11
+enum   = true
+union  = 5
+bare   = 1

examples/expected/0155-types-backtick-typed-const-union-tag.exit

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

examples/expected/0155-types-backtick-typed-const-union-tag.stderr

@@ -0,0 +1 @@
+

examples/expected/0155-types-backtick-typed-const-union-tag.stdout

@@ -0,0 +1,3 @@
+typed const = 5
+union tick  = 42
+union bare  = 42

examples/expected/1139-diagnostics-backtick-raw-not-a-type.stderr

@@ -1,5 +0,0 @@
-error: `s2` is a raw identifier, not a type — the backtick escape names a value, never a type
-  --> examples/1139-diagnostics-backtick-raw-not-a-type.sx:10:10
-   |
-10 |     x : `s2 = 1;
-   |          ^^

examples/expected/1141-diagnostics-reserved-name-type-decl.stderr

@@ -0,0 +1,29 @@
+error: 's8' is a reserved type name and cannot be used as an identifier
+  --> examples/1141-diagnostics-reserved-name-type-decl.sx:14:1
+   |
+14 | s8 :: struct { v: s64; }
+   | ^^
+
+error: 's16' is a reserved type name and cannot be used as an identifier
+  --> examples/1141-diagnostics-reserved-name-type-decl.sx:15:1
+   |
+15 | s16 :: enum { A; B; }
+   | ^^^
+
+error: 'u16' is a reserved type name and cannot be used as an identifier
+  --> examples/1141-diagnostics-reserved-name-type-decl.sx:16:1
+   |
+16 | u16 :: union { a: s32; b: f32; }
+   | ^^^
+
+error: 'u32' is a reserved type name and cannot be used as an identifier
+  --> examples/1141-diagnostics-reserved-name-type-decl.sx:17:1
+   |
+17 | u32 :: error { Bad, Empty }
+   | ^^^
+
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> examples/1141-diagnostics-reserved-name-type-decl.sx:18:1
+   |
+18 | s2 : s64 : 5;
+   | ^^

examples/expected/1141-diagnostics-reserved-name-type-decl.stdout

@@ -0,0 +1 @@
+