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/sema.zig

@@ -1295,7 +1295,6 @@ pub const Analyzer = struct {
             .inferred_type,
             .builtin_expr,
             .compiler_expr,
-            .foreign_expr,
             .library_decl,
             .framework_decl,
             .function_type_expr,
@@ -1761,7 +1760,6 @@ pub fn findNodeAtOffset(node: *Node, offset: u32) ?*Node {
         .inferred_type,
         .builtin_expr,
         .compiler_expr,
-        .foreign_expr,
         .library_decl,
         .framework_decl,
         .function_type_expr,