refactor(ffi-linkage): Phase 9.1d — eliminate the foreign_expr AST node

The last linkage-family 'foreign' carrier. Migrated c_import.zig auto-synthesis
(#import c {#include}) to build the extern shape (empty-block body + extern_export
= .extern_) instead of a foreign_expr body — the Phase 5.0 fn-body flip applied to
auto-synth. With nothing left building it, deleted the foreign_expr union variant +
ForeignExpr struct (ast.zig) and every reader: the dead-arm switch cases (sema,
resolver, generic, call, semantic_diagnostics, lsp), the coalescing reads in
decl.zig (is_foreign local, cc/rename/dedup/variadic/visibility gates) + pack.zig,
and checkForeignRefs (now reads extern_lib only). 9.1 LINKAGE PURGE COMPLETE — all
that remains in src/ is the runtime-class family (9.2) + comments. Snapshot-neutral
(the #import c examples 1215/1216/1217 + sqlite 1624 exercise the synth path); suite
green (646 corpus / 444 unit, 0 failed).

src/ir/resolver.zig

@@ -184,7 +184,7 @@ pub fn isPlainFreeFnDecl(fd: *const ast.FnDecl) bool {
     // body, so it stays plain-free.
     if (fd.extern_export == .extern_) return false;
     return switch (fd.body.data) {
-        .foreign_expr, .builtin_expr, .compiler_expr => false,
+        .builtin_expr, .compiler_expr => false,
         else => true,
     };
 }