refactor(ffi-linkage): Phase 9.3-src — purge 'foreign' from src/ comments + a user-facing diagnostic

Reword every 'foreign' comment to the extern/runtime-class vocabulary matching the
renamed identifiers (foreign call→extern call, foreign class→runtime class, foreign
path→runtime path, the #foreign-literal comment mentions → extern, etc.). Also fixes
two USER-FACING issues: the 'expected … #foreign … after type annotation' parse error
no longer advertises the removed keyword, and the Android 'no #jni_main' help
diagnostic now shows '#jni_class(…) extern' instead of the rejected '#foreign
#jni_class'. Removed the now-dead prefix-#foreign-vs-postfix conflict branch in
parseRuntimeClassDecl (the caller rejects #foreign before it runs).

src/ now contains 'foreign' ONLY in the hash_foreign token machinery + its 4
rejection messages — the deprecation mechanism (kept per the 9.0 recommendation; the
message MUST name #foreign to guide migration). Snapshot-neutral; suite green
(646 corpus / 444 unit, 0 failed).

src/backend/llvm/ops.zig

@@ -485,7 +485,7 @@ pub const Ops = struct {
     pub fn emitObjcMsgSend(self: Ops, instruction: *const Inst, msg: ObjcMsgSend) void {
         const msg_send = self.e.getObjcMsgSendValue();
         // Detect the sret case: >16 B non-HFA struct return.
-        // Same predicate as the plain-foreign-call path so the
+        // Same predicate as the plain-extern-call path so the
         // two arms stay in lockstep.
         const raw_ret_ty = self.e.toLLVMType(instruction.ty);
         const uses_sret = self.e.needsByval(instruction.ty, raw_ret_ty);
@@ -516,7 +516,7 @@ pub const Ops = struct {
         call_args[sret_off + 1] = self.e.coerceArg(self.e.resolveRef(msg.sel), self.e.cached_ptr);
         // additional args take their IR types, with ABI
         // coercion applied so structs / strings decay the
-        // same way they do for any C foreign call.
+        // same way they do for any C extern call.
         for (msg.args, 0..) |arg_ref, i| {
             const raw_ty = self.e.argIRTypeOrFail(arg_ref);
             const raw_llvm = self.e.toLLVMType(raw_ty);