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/objc_class.zig

@@ -53,7 +53,7 @@ pub fn lowerObjcDefinedClassMethods(self: *Lowering) void {
     self.emitObjcDefinedClassImps();
 }
 
-/// If `obj_expr` is typed as a pointer to a foreign Obj-C class
+/// If `obj_expr` is typed as a pointer to a runtime Obj-C class
 /// and that class (or any of its `#extends` ancestors) declares a
 /// `#property` field with the given name, return the
 /// `RuntimeFieldDecl`. M2.2 + M2.3.
@@ -257,7 +257,7 @@ pub fn lowerObjcPropertySetter(self: *Lowering, obj_expr: *const ast.Node, field
 
 /// Get a FuncId for an external C-callconv function. If a function
 /// with this exported name already exists in the module (e.g.
-/// declared by stdlib `#foreign` decl), return it; otherwise
+/// declared by stdlib `extern` decl), return it; otherwise
 /// declare it fresh with the given signature.
 ///
 /// One helper instead of a `get<Name>Fid` per runtime function —
@@ -729,7 +729,7 @@ pub fn emitObjcDefinedClassImp(self: *Lowering, fcd: *const ast.RuntimeClassDecl
 
     // Pass the Obj-C receiver pointer through to the sx body as
     // `self`. The body's `self: *Self` type resolves to the
-    // foreign-class stub (the opaque Obj-C type), matching Apple's
+    // runtime-class stub (the opaque Obj-C type), matching Apple's
     // Obj-C semantics where `self` IS the object. `self.field`
     // access on a sx-defined class is rewritten by lowerFieldAccess
     // to go through `object_getIvar(self, __sx_state_ivar)` and