ffi 3.1: Cls.static_method(args) lowers to objc_msg_send on the class object

Implementation half of the Phase 3.1 cadence step.
`lowerForeignStaticCall` for `#objc_class` / `#objc_protocol` runtimes
no longer bails; it routes through a new `lowerObjcStaticCall` helper
that loads the class object from a module-scoped cached slot (populated
once per module via `objc_getClass`) and dispatches `objc_msg_send`
with the same selector-mangling as Phase 3.0's instance dispatch.

Three pieces:

1. `Module.objc_class_cache` — parallel to `objc_selector_cache`,
   insertion-ordered list of (class_name, slot_GlobalId) so the
   constructor that calls `objc_getClass` per slot at module load
   is deterministic. `lookupObjcClass` / `appendObjcClass` accessors.
2. `internObjcClassObject` in lower.zig — get-or-create a
   `OBJC_CLASSLIST_REFERENCES_<Cls>` global pointer; matches clang's
   naming convention. `lowerObjcStaticCall` reuses
   `deriveObjcSelector` from 3.0 for the selector, loads the class
   slot, and emits `objc_msg_send(class_obj, sel, args)`.
3. `emitObjcClassInit` in emit_llvm.zig — companion to
   `emitObjcSelectorInit`. Walks `objc_class_cache`, synthesizes a
   constructor `__sx_objc_class_init` that calls `objc_getClass(name)`
   per slot, registers in `@llvm.global_ctors` for AOT (extending the
   existing array if the selector init already created it), and
   injects a direct call into main's prelude after any prior init
   calls so the ORC JIT path runs it too.

Surface form is `.` (`NSObject.class()`) matching JNI's `Alias.new(...)`
convention rather than the plan's notional `::` — avoids extending the
parser for a new postfix operator with no other use case.

Test `examples/ffi-objc-dsl-05-static.sx` exercises NSObject's
`+class` and `+description` class methods via the new syntax, asserts
both return non-null. NSObject is always available at module-load,
unlike runtime-created test classes that wouldn't exist yet when
the class-init constructor runs.

164/164 tests; chess builds + runs clean on all three platforms.

current/CHECKPOINT-FFI.md

@@ -485,12 +485,23 @@ override (3.2). New helpers `deriveObjcSelector` and
 working output (and the mismatch case to the specific keyword-count
 error).
 
-Open work, in roughly the order they make sense:
+Phase 3 step 3.1 landed: `Cls.static_method(args)` on an `#objc_class`
+alias loads the class object through a module-scoped cached slot
+(`OBJC_CLASSLIST_REFERENCES_<Cls>`, populated once per module via
+`objc_getClass` at module-init) and dispatches `objc_msg_send` with
+the same selector derivation as 3.0. New `Module.objc_class_cache`
+parallel to `objc_selector_cache`; `internObjcClassObject` and
+`lowerObjcStaticCall` helpers in lower.zig; `emitObjcClassInit`
+constructor in emit_llvm.zig that walks the cache, runs
+`objc_getClass` per slot, registers via `@llvm.global_ctors`, and
+injects a direct call into `main` for the ORC JIT path. Surface form
+is `.` (matching JNI's `Alias.new(...)` convention) rather than the
+plan's notional `::` — avoids a new postfix operator. Test:
+`examples/ffi-objc-dsl-05-static.sx` — exercises NSObject's `+class`
+and `+description` class methods (NSObject is always available at
+module-load, unlike test classes created in main's body).
 
-- **Phase 3 step 3.1** — static call `Cls::class_method(args)` lowers
-  to `#objc_call` on the class object (loaded via `objc_getClass` once
-  and interned per module). Same pattern as 3.0 for the niladic /
-  arity-N selector derivation; the new piece is the class-object slot.
+Open work, in roughly the order they make sense:
 - **Phase 3 step 3.2** — `#selector("explicit:")` override + golden
   test for the default-mangling table. Escape hatch for selectors
   that don't fit the underscore-split rule (e.g. `tableView_