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/lower/call.zig

@@ -726,7 +726,7 @@ pub fn lowerCall(self: *Lowering, c_in: *const ast.Call) Ref {
                             // literal global symbol — the existing bare-name
                             // machinery below resolves them.
                             switch (fd.body.data) {
-                                .foreign_expr, .builtin_expr, .compiler_expr => break :gate,
+                                .builtin_expr, .compiler_expr => break :gate,
                                 else => {},
                             }
                             if (hasComptimeParams(fd)) return self.lowerComptimeCall(fd, c);