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/jni_java_emit.test.zig

@@ -230,7 +230,7 @@ test "default-package class (no slash in runtime_path)" {
     const out = try emit.emitJavaSource(a, &fcd, .{});
     defer a.free(out);
 
-    // No `package ...;` line when the foreign path has no slashes.
+    // No `package ...;` line when the runtime path has no slashes.
     try std.testing.expect(std.mem.indexOf(u8, out, "package ") == null);
     try std.testing.expect(std.mem.indexOf(u8, out, "public class SxNoPackage") != null);
 }