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).

src/ir/inst.zig

@@ -456,6 +456,12 @@ pub const Function = struct {
     /// sites apply the standard default argument promotions (s8/s16/bool →
     /// s32, f32 → f64) to extras past the fixed param count.
     is_variadic: bool = false,
+    /// True if `params[0]` is the synthetic `__sx_ctx: *Context`
+    /// parameter that every default-conv sx function receives. Callers
+    /// read this flag to decide whether to prepend their current
+    /// `__sx_ctx` value to the args of a call. Foreign decls and
+    /// `callconv(.c)` functions have it false.
+    has_implicit_ctx: bool = false,
 
     pub const Param = struct {
         name: StringId,