refactor(ffi-linkage): Phase 9.2b — rename runtime-class fns + state → runtime_* / is_reference

The runtime-class object-model identifiers (Decision 5): parse/lower/find/resolve/
register/stamp fns Foreign→Runtime (parseRuntimeClassDecl, lowerRuntimeMethodCall,
findRuntimeMethodInChain, resolveRuntimeMethodReturnType, registerRuntimeClassDecl,
runtimeClassStructType, runtimeKindForOffset, …); state foreign_class_map→
runtime_class_map, current_foreign_class/_method→current_runtime_*, the
foreign_class_decl union variant→runtime_class_decl, foreign_method/static/instance/
class→runtime_*; and the reference-vs-define flag is_foreign→is_reference (+
is_foreign_eff→is_reference_eff) now that it only lives on RuntimeClassDecl.
Snapshot-neutral; suite green (646/444).

Remaining 9.2: the foreign_path family (coupled .sx hooks: jni_main_foreign_path_at
spans build.sx/bundle.sx/compiler_hooks.zig/specs.md) + the extern-ref validators
(checkForeignRefs etc. → Extern, linkage not runtime) + bare 'foreign' comments.

src/backend/llvm/ffi_ctors.zig

@@ -295,7 +295,7 @@ pub const FfiCtors = struct {
             // Parent class — pre-resolved Obj-C runtime name from
             // lower.zig (M2.3 resolveObjcParentName). Stored on the
             // cache entry so emit_llvm doesn't re-walk
-            // foreign_class_map here.
+            // runtime_class_map here.
             const parent_name = entry_kv.parent_objc_name;
 
             const parent_str_global = self.e.emitPrivateCString(parent_name, "OBJC_CLASS_NAME_");