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/module.zig

@@ -43,7 +43,7 @@ pub const Module = struct {
     /// the class object through this cache once per module.
     objc_class_cache: std.ArrayList(ObjcClassEntry),
     /// sx-defined Obj-C classes — every `Cls :: #objc_class("Cls") { ... }`
-    /// declaration WITHOUT `#foreign`. Insertion-ordered so the
+    /// declaration WITHOUT `extern`. Insertion-ordered so the
     /// class-registration constructors (M1.2 A.4) emit in source order
     /// — parent classes register before children, which matters because
     /// `objc_allocateClassPair(super, ...)` resolves `super` by lookup.