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.zig

@@ -24,7 +24,7 @@
 //     f32/f64)
 //   - `(self: *Self)` plus primitive params
 //   - cross-class refs (`*Foo` where Foo is another declared
-//     `#foreign #jni_class`) lower to Foo's foreign path → Java
+//     `#jni_class(…) extern`) lower to Foo's runtime path → Java
 //     fully-qualified type
 //   - `*void` → `Object` (opaque jobject)
 
@@ -39,7 +39,7 @@ pub const EmitError = error{
 };
 
 pub const Options = struct {
-    /// Map from sx alias → foreign path of declared `#jni_class` decls.
+    /// Map from sx alias → runtime path of declared `#jni_class` decls.
     /// Used to resolve `*Foo` cross-class refs in method signatures.
     classes: ?*const std.StringHashMap([]const u8) = null,
     /// Default superclass when the user doesn't write `#extends ...;`.
@@ -79,7 +79,7 @@ pub fn injectLoadLibrary(allocator: Allocator, java_source: []const u8, lib_name
     return try buf.toOwnedSlice(allocator);
 }
 
-/// Emit a `.java` source for the given foreign-class decl. Result is
+/// Emit a `.java` source for the given runtime-class decl. Result is
 /// heap-allocated through `allocator`; caller owns it.
 pub fn emitJavaSource(
     allocator: Allocator,