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/ir/ffi_objc.zig

@@ -172,7 +172,7 @@ pub const ObjcLowering = struct {
             .isize => try out.append(self.l.alloc, 'q'),
             .usize => try out.append(self.l.alloc, 'Q'),
             .pointer => |p| {
-                // Pointer to a foreign Obj-C class (or sx-defined #objc_class)
+                // Pointer to a runtime Obj-C class (or sx-defined #objc_class)
                 // encodes as `@`. Anything else falls to `^v` — generic
                 // pointer; the runtime treats it as opaque.
                 const pointee_info = self.l.module.types.get(p.pointee);
@@ -378,7 +378,7 @@ pub const ObjcLowering = struct {
             const pointee = self.l.module.types.get(field_ty).pointer.pointee;
             // `*void` is NOT considered an object pointer — ambiguous.
             if (pointee == .void) break :blk false;
-            // `*T` where T is a foreign-class struct (Obj-C class).
+            // `*T` where T is a runtime-class struct (Obj-C class).
             if (pointee.isBuiltin()) break :blk false;
             const pointee_info = self.l.module.types.get(pointee);
             if (pointee_info != .@"struct") break :blk false;