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/lower/expr.zig

@@ -567,7 +567,7 @@ pub fn lowerFieldAccess(self: *Lowering, fa: *const ast.FieldAccess, span: ast.S
     }
 
     // M2.2 — `obj.field` where `field` is declared with `#property`
-    // on a foreign Obj-C class lowers as `[obj field]` (the synthesized
+    // on a runtime Obj-C class lowers as `[obj field]` (the synthesized
     // getter). Receiver stays opaque — no auto-deref.
     if (self.lookupObjcPropertyOnPointer(fa.object, fa.field)) |prop| {
         return self.lowerObjcPropertyGetter(fa.object, prop, fa.field, span);