mem: Step 3 — thread __sx_ctx through closure/fn-pointer/method dispatch

Continues the implicit-Context refactor. Bare-fn trampolines, lambda
trampolines, and protocol thunks now carry __sx_ctx at slot 0; call
sites for closures, fn-pointer variables, and method dispatch prepend
the caller's current ctx.

- emit_llvm.zig:1687 call_indirect treats `fp_ctx_slots` leading args
  as opaque ptr (the implicit ctx) when the fn-pointer is default-conv
  under has_implicit_ctx.
- lower.zig:fnPtrTypeWantsCtx predicate gates the prepend at both
  scope-local and global fn-pointer call sites.
- lower.zig:fixupMethodReceiver skips __sx_ctx when probing the
  receiver param's type.
- lower.zig:lowerLambda builds closure type from user-visible params
  only (skip ctx + env).
- lower.zig:closure(bare_fn) builds closure type from user-visible
  params only.
- module.zig: Module.has_implicit_ctx flag mirrors Lowering's switch
  so emit_llvm can read it without a back-pointer.

Tests updated:
- 5 ObjC-block/runtime tests get `callconv(.c)` on fn-ptr types
  cast from `objc_msgSend` / Block.invoke (C-side calls into sx).
- ffi-06-callback gets `callconv(.c)` on double_it/add_with_ctx —
  the registered C-side callbacks.
- 08-types snapshot regen (undefined-init drift from layout shift).
- 11 JNI/ObjC .ir snapshots regen for the ctx-prepended thunk
  signatures.

151/152 example tests pass. Remaining failure (05-run) is the
comptime/interp path that requires Step 7 (callWithDefaultContext).

examples/61-objc-roundtrip.sx

@@ -18,10 +18,10 @@ main :: () -> s32 {
     sel_with_utf8 := sel_registerName("stringWithUTF8String:".ptr);
     sel_utf8      := sel_registerName("UTF8String".ptr);
 
-    msg_3 : (*void, *void, [*]u8) -> *void = xx objc_msgSend;
+    msg_3 : (*void, *void, [*]u8) -> *void callconv(.c) = xx objc_msgSend;
     ns_str := msg_3(ns_class, sel_with_utf8, "hi".ptr);
 
-    msg_2 : (*void, *void) -> [*]u8 = xx objc_msgSend;
+    msg_2 : (*void, *void) -> [*]u8 callconv(.c) = xx objc_msgSend;
     back := msg_2(ns_str, sel_utf8);
 
     return xx (back[0] + back[1]); // 'h' + 'i' = 104 + 105 = 209