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/emit_llvm.zig

@@ -589,14 +589,11 @@ pub const LLVMEmitter = struct {
             const fcd = entry_kv.decl;
             const class_name = fcd.name;
 
-            // Parent class — find `.extends` member; default NSObject.
-            var parent_name: []const u8 = "NSObject";
-            for (fcd.members) |m| {
-                switch (m) {
-                    .extends => |p| parent_name = p,
-                    else => {},
-                }
-            }
+            // 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.
+            const parent_name = entry_kv.parent_objc_name;
 
             const parent_str_global = self.emitPrivateCString(parent_name, "OBJC_CLASS_NAME_");
             const class_str_global = self.emitPrivateCString(class_name, "OBJC_CLASS_NAME_");