ffi M4.A: stdlib NSObject + autoreleasepool helper + extends rooting

Declare `NSObject` in std/objc.sx as `#foreign #objc_class("NSObject")`
with the canonical instance + class-method surface every Obj-C class
inherits: `retain`/`release`/`autorelease`/`new`/`alloc`/`init`/
`description`/`hash`/`isEqual_`/`isKindOfClass_`/`respondsToSelector_`/
`class`. Root the foreign-class hierarchy in uikit.sx at NSObject by
adding `#extends NSObject;` to every previously-unrooted declaration
(NSValue, NSNumber, NSDictionary, NSSet, NSNotification, NSBundle,
NSNotificationCenter, NSRunLoop, CADisplayLink, CALayer, EAGLContext,
UIScreen, UIResponder) plus deeper chain fixes (NSMutableDictionary
extends NSDictionary; UIWindow extends UIView; UIViewController
extends UIResponder). After this, M2.3's extends-chain walk finds
`retain`/`release` on any UIKit-typed value:

  view := UIView.alloc().init();
  defer view.release();        // canonical sx idiom — no language magic

Plus `autoreleasepool(body: Closure())` stdlib helper that wraps
`body` in `objc_autoreleasePoolPush` / `defer objc_autoreleasePoolPop`.
Required for Foundation factory returns; closure-call frame is real
cost so hot loops should inline the push/defer-pop pattern manually.

Smoke test `ffi-objc-arc-01-autoreleasepool.sx` exercises both
patterns; refresh of two IR snapshots picks up the new stdlib decls
appearing in test outputs that include `modules/std/objc.sx`.

185/185 example tests pass; chess on iOS-sim green.

library/modules/std/objc.sx

@@ -93,3 +93,50 @@ c_string :: (ns: *void) -> [*]u8 {
     fn_ptr : (*void, *void) -> [*]u8 callconv(.c) = xx objc_msgSend;
     return fn_ptr(ns, sel);
 }
+
+// ─── NSObject (Phase 4 / M4.A) ───────────────────────────────────────────
+// Root of every Obj-C class hierarchy. Apple's runtime supplies the IMPs;
+// sx declares the method surface so user code can write
+// `defer view.release();` and `view.retain()` directly instead of going
+// through `objc_msgSend` casts. M2.3's `#extends`-aware dispatch finds
+// these methods automatically once a class roots its `#extends` chain at
+// NSObject (foreign classes in uikit.sx etc. add `#extends NSObject;` to
+// inherit the surface).
+//
+// `release` is NOT a reserved keyword in sx — modern clang ARC bans
+// user-source calls to it (the ARC compiler emits them automatically), but
+// sx isn't under clang ARC. Calling `view.release()` here is equivalent to
+// pre-ARC Obj-C source code: dispatches through the runtime, decrements the
+// refcount, fires `-dealloc` at zero.
+NSObject :: #foreign #objc_class("NSObject") {
+    alloc       :: () -> *NSObject;
+    init        :: (self: *Self) -> *NSObject;
+    new         :: () -> *NSObject;       // shorthand for [[Cls alloc] init]
+    retain      :: (self: *Self) -> *Self;
+    release     :: (self: *Self);
+    autorelease :: (self: *Self) -> *Self;
+    class       :: () -> *void;            // metaclass query — `Cls.class()`
+    description :: (self: *Self) -> *void;  // returns *NSString
+    hash        :: (self: *Self) -> u64;
+    isEqual_    :: (self: *Self, other: *void) -> BOOL;
+    isKindOfClass_ :: (self: *Self, cls: *void) -> BOOL;
+    respondsToSelector_ :: (self: *Self, sel: *void) -> BOOL;
+}
+
+// ─── Autoreleasepool (M4.A) ──────────────────────────────────────────────
+// Foundation factory methods (`NSString.stringWithUTF8String:`,
+// `[NSArray array]`, ...) return autoreleased objects — valid until the
+// current pool drains. Wrap such code in `autoreleasepool(() => { ... })`
+// so the pool drains deterministically at block end.
+//
+// Stdlib helper, not a language keyword. The closure call adds a frame —
+// for hot loops, inline the push/defer-pop pattern manually.
+
+objc_autoreleasePoolPush :: ()                  -> *void  #foreign objc;
+objc_autoreleasePoolPop  :: (pool: *void)                 #foreign objc;
+
+autoreleasepool :: (body: Closure()) {
+    pool := objc_autoreleasePoolPush();
+    defer objc_autoreleasePoolPop(pool);
+    body();
+}