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,