ffi M2.3: #extends method-resolution chaining + Obj-C parent resolution

When 'obj.method()' is called on a foreign-class pointer and the
method isn't declared on the receiver's class, the compiler walks
the '#extends' chain to find an ancestor that declared it.
Property lookup (M2.2) flows through the same chain walker.

  ParentX :: #foreign #objc_class("...") { foo :: ... }
  ChildX  :: #foreign #objc_class("...") { #extends ParentX; }

  child.foo()   // now resolves — was 'no method foo on ChildX'

Two new helpers in lower.zig:
- findForeignMethodInChain(fcd, name) walks the cache via
  fcd.members[i].extends → foreign_class_map[parent] → ...
  Depth-capped at 16 to break accidental cycles.
- findForeignPropertyInChain(fcd, name) — same shape for fields.

ALSO fixes a latent class-hierarchy bug uncovered while testing
M2.3: emit_llvm was passing the sx alias name to
objc_allocateClassPair(super, ...) rather than the actual Obj-C
runtime class name. For 'SxThing :: #objc_class(...) { #extends
NSObjectBase; }' where 'NSObjectBase' is aliased to "NSObject",
emit_llvm produced 'objc_getClass("NSObjectBase")' → NULL →
'objc_allocateClassPair(NULL, ...)' → SxThing's super-class link
was broken → '[sx_thing hash]' bypassed NSObject and crashed in
the forwarding machinery.

Fix: ObjcDefinedClassEntry gains a 'parent_objc_name' field
pre-resolved by lower.zig's 'resolveObjcParentName' through
foreign_class_map (which has the alias → foreign_path mapping).
emit_llvm just reads the resolved name from the entry.

153-objc-extends-chain.sx exercises both fixes:
  1-level: SxThing → NSObject — t.hash() walks one #extends.
  2-level: SxLeaf  → SxMiddle → NSObject — chained #extends.
Both return real NSObject.hash values from libobjc.

183 example tests pass (+1). zig build test green.

src/ir/module.zig

@@ -70,6 +70,12 @@ pub const Module = struct {
         name: []const u8,
         decl: *const ast.ForeignClassDecl,
         methods: []const ObjcDefinedMethodEntry = &.{},
+        /// Pre-resolved Obj-C runtime name of the parent class, so
+        /// emit_llvm can pass it to `objc_getClass(parent)` /
+        /// `objc_allocateClassPair(super, ...)` without walking the
+        /// sx-side foreign_class_map (which lives in lower.zig).
+        /// Defaults to "NSObject" when no `#extends` member is present.
+        parent_objc_name: []const u8 = "NSObject",
     };
 
     pub const ObjcDefinedMethodEntry = struct {
@@ -155,6 +161,16 @@ pub const Module = struct {
         }
     }
 
+    /// Set the resolved Obj-C runtime parent name on a cache entry.
+    pub fn setObjcDefinedClassParent(self: *Module, name: []const u8, parent_objc_name: []const u8) void {
+        for (self.objc_defined_class_cache.items) |*entry| {
+            if (std.mem.eql(u8, entry.name, name)) {
+                entry.parent_objc_name = parent_objc_name;
+                return;
+            }
+        }
+    }
+
     pub fn addFunction(self: *Module, func: Function) FuncId {
         const id = FuncId.fromIndex(@intCast(self.functions.items.len));
         self.functions.append(self.alloc, func) catch unreachable;