refactor(ffi-linkage): Phase 9.1a — rename collision-free linkage identifiers

Mechanical src/ rename of the linkage-family identifiers whose extern_* target is
collision-free: callForeign→callExtern, marshalForeignArg→marshalExternArg,
dedupeForeignSymbol→dedupeExternSymbol, foreign_name_map→extern_name_map,
is_foreign_c_api→is_extern_c_api. Snapshot-neutral (internal only); suite green
(646 corpus / 444 unit, 0 failed).

Deferred (need per-site analysis — target name already exists): is_foreign↔is_extern
(38 existing), foreign_lib/foreign_name↔extern_lib/extern_name (15/16 existing),
foreign_expr (still built by c_import.zig auto-synthesis). Runtime-class family
(ForeignClassDecl etc. → Runtime*, Decision 5) is Phase 9.2.

src/ir/lower/call.zig

@@ -1255,7 +1255,7 @@ pub fn allocViaContext(self: *Lowering, size_ref: Ref, void_ptr_ty: TypeId) Ref
 /// Used for the compiler-internal byte-copy in the protocol-erasure
 /// heap path and the closure env-copy path, both of which need
 /// libc `memcpy` after the `#builtin` form was dropped.
-pub fn callForeign(self: *Lowering, name: []const u8, args: []const Ref, ret_ty: TypeId) Ref {
+pub fn callExtern(self: *Lowering, name: []const u8, args: []const Ref, ret_ty: TypeId) Ref {
     const fid = self.resolveFuncByName(name) orelse @panic("foreign symbol missing — std.sx not imported?");
     return self.builder.call(fid, args, ret_ty);
 }
@@ -1286,7 +1286,7 @@ fn protocolHasMethod(proto_info: anytype, name: []const u8) bool {
 
 pub fn resolveFuncByName(self: *Lowering, name: []const u8) ?FuncId {
     // Check foreign name map first (e.g., "c_abs" → "abs")
-    const effective_name = self.foreign_name_map.get(name) orelse name;
+    const effective_name = self.extern_name_map.get(name) orelse name;
     const name_id = self.module.types.internString(effective_name);
     for (self.module.functions.items, 0..) |func, i| {
         if (func.name == name_id) return FuncId.fromIndex(@intCast(i));