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/calls.test.zig

@@ -7,7 +7,7 @@
 //   2. The `CallPlan` object built by `CallResolver.plan` — its selected
 //      kind / target / variant and the receiver / `__sx_ctx` / default-arg
 //      properties, across every call form pinned by A3.2 sub-step 1
-//      (direct / UFCS / protocol / closure / fn-pointer / foreign / enum /
+//      (direct / UFCS / protocol / closure / fn-pointer / extern / enum /
 //      namespace). `resultType` is just `plan(c).return_type`, so these also
 //      lock the typing the regression suite relies on.
 
@@ -346,7 +346,7 @@ test "plan: struct (UFCS) method via #compiler dispatch + prepends receiver" {
     try std.testing.expect(p.prepends_receiver);
 }
 
-test "plan: foreign-class instance vs static dispatch" {
+test "plan: runtime-class instance vs static dispatch" {
     var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
     defer arena.deinit();
     const alloc = arena.allocator();