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_descriptor.zig

@@ -16,8 +16,8 @@
 //   f64           → D   (jdouble)
 //   []T           → [<elem>
 //   [*]T          → [<elem>   (sx many-pointer treated as array for now)
-//   *Self         → L<enclosing-foreign-path>;
-//   *Foo          → L<Foo's foreign path>;   (cross-class — step 2.9)
+//   *Self         → L<enclosing-runtime-path>;
+//   *Foo          → L<Foo's runtime path>;   (cross-class — step 2.9)
 //
 // `#jni_method_descriptor("...")` (step 2.6) overrides this whole walk
 // when set; sema/lowering use the override verbatim.
@@ -36,7 +36,7 @@ pub const DeriveError = error{
     OutOfMemory,
 };
 
-/// Map from sx-side alias → foreign path of declared `#jni_class` /
+/// Map from sx-side alias → runtime path of declared `#jni_class` /
 /// `#jni_interface` decls. Used to resolve `*Foo` into `L<path>;` in
 /// the descriptor. Built during lowering's scan pass.
 pub const ClassRegistry = std.StringHashMap([]const u8);
@@ -81,7 +81,7 @@ pub fn writeType(
             try writeType(allocator, buf, ctx, arr.element_type);
         },
         .pointer_type_expr => |ptr| {
-            // *Self → L<enclosing>;,  *Foo → L<Foo's foreign path>;,
+            // *Self → L<enclosing>;,  *Foo → L<Foo's runtime path>;,
             // *void → Ljava/lang/Object; (opaque jobject — common when
             // users don't have a precise Java type for the value).
             const inner = ptr.pointee_type;