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/sema.zig

@@ -1332,16 +1332,16 @@ pub const Analyzer = struct {
                     }
                 }
             },
-            .foreign_class_decl => |fd| {
+            .runtime_class_decl => |fd| {
                 try self.addSymbol(fd.name, .type_alias, null, node.span);
-                if (fd.is_foreign and fd.is_main) {
+                if (fd.is_reference and fd.is_main) {
                     try self.diagnostics.append(self.allocator, .{
                         .level = .err,
                         .message = "'#foreign' and '#jni_main' / '#objc_main' are mutually exclusive — a foreign-referenced class can't be the app's main entry",
                         .span = node.span,
                     });
                 }
-                if (fd.is_foreign) {
+                if (fd.is_reference) {
                     for (fd.members) |m| switch (m) {
                         .method => |md| if (md.body != null) {
                             try self.diagnostics.append(self.allocator, .{
@@ -1784,7 +1784,7 @@ pub fn findNodeAtOffset(node: *Node, offset: u32) ?*Node {
         .tuple_type_expr,
         .ufcs_alias,
         .closure_type_expr,
-        .foreign_class_decl,
+        .runtime_class_decl,
         => {},
         .jni_env_block => |eb| {
             if (findNodeAtOffset(eb.env, offset)) |found| return found;