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.

tests/expected/ffi-objc-dsl-05-static.exit

@@ -1 +1 @@
-1
+0

tests/expected/ffi-objc-dsl-05-static.txt

@@ -1,2 +1,2 @@
-/Users/agra/projects/sx/examples/ffi-objc-dsl-05-static.sx:41:14: error: static calls on 'objc_class' runtime not yet supported (Phase 3/4)
-/Users/agra/projects/sx/examples/ffi-objc-dsl-05-static.sx:43:14: error: static calls on 'objc_class' runtime not yet supported (Phase 3/4)
+class non-null
+description non-null