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/lsp/server.zig

@@ -1325,7 +1325,7 @@ pub const Server = struct {
                 // Skip functions, types, structs, enums, unions, comptime, foreign, library
                 switch (cd.value.data) {
                     .fn_decl, .type_expr, .struct_decl, .enum_decl, .union_decl,
-                    .comptime_expr, .foreign_expr, .library_decl,
+                    .comptime_expr, .library_decl,
                     => return,
                     else => {},
                 }