ffi 3.2 C2: migrate uikit.sx Notifications + Bundle cluster

Second cluster: NSNotification, NSBundle, NSNotificationCenter move
from `#objc_call(T)(recv, "sel:", args)` to declarative
`#foreign #objc_class("Cls") { ... }` blocks.

Classes declared (alongside the C1 Foundation utility group):

- NSNotification        → userInfo (instance, returns *NSDictionary)
- NSBundle              → mainBundle (class), resourcePath (instance)
- NSNotificationCenter  → defaultCenter (class),
                          addObserver_selector_name_object (instance)

The 4-keyword `addObserver:selector:name:object:` selector derives
from the underscore-separated sx name via the default mangling rule
— no `#selector` override needed.

Cleanup wins:

- `objc_getClass("NSBundle")` and `objc_getClass("NSNotificationCenter")`
  call sites gone — class slots now populated by emit_llvm's
  `__sx_objc_class_init` constructor.
- `userInfo`'s return type is `*NSDictionary` directly, so the
  previous `*void → *NSDictionary` cast at the keyboard-frame
  callsite collapses.

166/166 tests; chess builds clean on macOS + iOS + Android via
`sx build main.sx`.

current/CHECKPOINT-FFI.md

@@ -553,8 +553,15 @@ foreign-class pointer at the dispatch site. The `objc_getClass(...)`
 calls for these classes are gone — the class slot is now populated
 by emit_llvm's `__sx_objc_class_init` constructor (Phase 3.1).
 
+Phase 3.2 C2 landed: notifications + bundle cluster migrated.
+NSNotification (`userInfo`), NSBundle (`mainBundle`, `resourcePath`),
+NSNotificationCenter (`defaultCenter`, `addObserver_selector_name_object`)
+declared as `#foreign #objc_class` blocks. The 4-keyword
+`addObserver:selector:name:object:` selector derives cleanly from
+the underscore-separated sx name (`addObserver_selector_name_object`).
+
 Open work:
-- **Phase 3 step 3.2 — C2..C5** — uikit.sx migration, one cluster
+- **Phase 3 step 3.2 — C3..C5** — uikit.sx migration, one cluster
   per commit, chess regression after each.
   test for the default-mangling table. Escape hatch for selectors
   that don't fit the underscore-split rule (e.g. `tableView_