refactor(ffi-linkage): Phase 9.1c — delete dead VarDecl legacy foreign fields

VarDecl carried BOTH the legacy is_foreign/foreign_lib/foreign_name AND the new
is_extern/extern_lib/extern_name (parallel forms coalesced during the migration).
The global #foreign parse path now rejects, so the legacy trio is write-dead and
read in only 3 coalescing sites (decl.zig). Simplified those readers
(vd.extern_name orelse vd.name; vd.is_extern) and deleted the dead fields. Build
confirms no other setter/reader. Snapshot-neutral; suite green (646/444).

Remaining linkage (9.1): foreign_expr (25, still built by c_import.zig auto-synth)
+ ForeignClassDecl.is_foreign (runtime-class, → 9.2). Runtime-class family (9.2,
Decision 5) is the big remaining src/ rename.

src/ast.zig

@@ -361,16 +361,10 @@ pub const VarDecl = struct {
     name_span: Span,
     type_annotation: ?*Node,
     value: ?*Node,
-    is_foreign: bool = false,
-    foreign_lib: ?[]const u8 = null,
-    foreign_name: ?[]const u8 = null,
     /// `extern`-global form `g : T extern [LIB] ["csym"];` — a reference to a
-    /// global defined elsewhere (external linkage, resolved at link time). The
-    /// new extern-named surface; distinct from the legacy `#foreign` path above.
+    /// global defined elsewhere (external linkage, resolved at link time).
     /// `extern_lib` is the optional source-library reference and `extern_name`
-    /// the optional symbol-name override (mirroring foreign_lib/foreign_name, so
-    /// `extern` fully supersedes `#foreign`). Parsed in Phase 0.1; not consumed
-    /// by the var-decl path until Phase 1.2.
+    /// the optional symbol-name override.
     is_extern: bool = false,
     extern_lib: ?[]const u8 = null,
     extern_name: ?[]const u8 = null,

src/ir/lower/decl.zig

@@ -1114,9 +1114,8 @@ pub fn registerTopLevelGlobal(self: *Lowering, vd: *const ast.VarDecl) void {
     };
     // Foreign / extern globals reference a symbol defined in libSystem etc.
     // (`_NSConcreteStackBlock : *void #foreign;` or `… : *void extern;`). The C
-    // symbol name is the optional override (`extern_name`/`foreign_name`) or the
-    // sx name itself.
-    const sym_name = vd.extern_name orelse vd.foreign_name orelse vd.name;
+    // symbol name is the optional override (`extern_name`) or the sx name itself.
+    const sym_name = vd.extern_name orelse vd.name;
     const name_id = self.module.types.internString(sym_name);
     const init_val = self.globalInitValue(vd, var_ty);
     const gid = self.module.addGlobal(.{
@@ -1124,13 +1123,13 @@ pub fn registerTopLevelGlobal(self: *Lowering, vd: *const ast.VarDecl) void {
         .ty = var_ty,
         .init_val = init_val,
         .is_const = false,
-        .is_extern = vd.is_foreign or vd.is_extern,
+        .is_extern = vd.is_extern,
     });
     self.putGlobal(self.current_source_file, vd.name, .{ .id = gid, .ty = var_ty });
 }
 
 /// Serialize a top-level global's initializer into a static `ConstantValue`.
-/// Foreign globals (extern symbol) and value-less declarations carry no
+/// Extern globals (external symbol) and value-less declarations carry no
 /// payload — they default to zero/extern at link, which is correct. An
 /// identifier initializer that names a module constant is materialized from
 /// the recorded constant (`K : A : 42; g : A = K;` → 42); a
@@ -1138,7 +1137,7 @@ pub fn registerTopLevelGlobal(self: *Lowering, vd: *const ast.VarDecl) void {
 /// is rejected with a diagnostic rather than silently zero-initialized — a
 /// global has no run site for a dynamic initializer.
 pub fn globalInitValue(self: *Lowering, vd: *const ast.VarDecl, var_ty: TypeId) ?inst_mod.ConstantValue {
-    if (vd.is_foreign or vd.is_extern) return null;
+    if (vd.is_extern) return null;
     const v = vd.value orelse return null;
     return switch (v.data) {
         .undef_literal => .zeroinit,