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/ir/lower/nominal.zig

@@ -296,7 +296,7 @@ pub fn rawNamedTypePtr(ref: resolver_mod.RawDeclRef) ?*const anyopaque {
         .union_decl => |d| @ptrCast(d),
         .error_set_decl => |d| @ptrCast(d),
         .protocol_decl => |d| @ptrCast(d),
-        .foreign_class_decl => |d| @ptrCast(d),
+        .runtime_class_decl => |d| @ptrCast(d),
         .fn_decl, .const_decl, .var_decl, .namespace_decl => null,
     };
 }