ffi M5.A.2: drop hand-rolled __block_invoke_* impls + Into(Block) per-sig boilerplate

The compiler-synthesised trampoline path (previous commit) covers
every closure signature on demand; the hand-rolled stdlib impls
were only for two specific shapes (`Closure() -> void`,
`Closure(bool) -> void`) and are now strictly redundant.

Kept: the `Block` struct, `BlockDescriptor`, the
`_NSConcreteStackBlock` extern decl, and the shared
`__sx_block_descriptor` global. The compiler-emitted code
references all four; users still need to `#import
"modules/std/objc_block.sx";` to bring them into the module.

Removed: `__block_invoke_void`, `__block_invoke_bool`, and both
`impl Into(Block) for Closure(...) -> void` blocks. Replaced with
a comment block explaining how the compiler now handles the cast.

After this commit, `xx my_closure : Block` works for ANY closure
signature with no per-signature stdlib boilerplate. 189/189
example tests pass; chess on iOS-sim green.

library/modules/std/objc_block.sx

@@ -52,52 +52,17 @@ __sx_block_descriptor : BlockDescriptor = .{
     size = 48,
 };
 
-// Per-signature invoke trampolines. Each one reads sx_env + sx_fn from
-// its block_self argument and tail-calls the closure through a typed
-// fn-ptr cast. One per Apple block signature we support.
+// M5.A — `xx closure : Block` casts are handled by the compiler.
+// For every closure signature seen at a cast site, the compiler
+// synthesises:
+//   1. A C-ABI trampoline `__block_invoke_<sig>` matching Apple's
+//      `__block_literal.invoke` calling convention.
+//   2. Inline Block-struct construction at the cast site, with
+//      `invoke` pointing at the synthesised trampoline and
+//      `sx_env`/`sx_fn` taken from the closure value.
 //
-// Signature: `void (^)(void)` — no args, no return. The single most
-// common Apple block shape (UIView animation bodies, dispatch_async, etc).
-__block_invoke_void :: (block_self: *Block) callconv(.c) {
-    // `sx_fn` is the closure trampoline — an sx-side function with the
-    // implicit __sx_ctx at slot 0 and env at slot 1. We're a callconv(.c)
-    // entry, so the call site needs ctx prepended; the typed fn-pointer
-    // type stays default-conv to enable that.
-    typed_fn : (*void) -> void = xx block_self.sx_fn;
-    typed_fn(block_self.sx_env);
-}
-
-impl Into(Block) for Closure() -> void {
-    convert :: (self: Closure() -> void) -> Block {
-        .{
-            isa = @_NSConcreteStackBlock,
-            flags = 0,
-            reserved = 0,
-            invoke = xx @__block_invoke_void,
-            descriptor = xx @__sx_block_descriptor,
-            sx_env = self.env,
-            sx_fn = self.fn_ptr,
-        };
-    }
-}
-
-// Signature: `void (^)(BOOL)` — UIView animation completion handlers and
-// similar one-arg-bool callbacks.
-__block_invoke_bool :: (block_self: *Block, arg0: bool) callconv(.c) {
-    typed_fn : (*void, bool) -> void = xx block_self.sx_fn;
-    typed_fn(block_self.sx_env, arg0);
-}
-
-impl Into(Block) for Closure(bool) -> void {
-    convert :: (self: Closure(bool) -> void) -> Block {
-        .{
-            isa = @_NSConcreteStackBlock,
-            flags = 0,
-            reserved = 0,
-            invoke = xx @__block_invoke_bool,
-            descriptor = xx @__sx_block_descriptor,
-            sx_env = self.env,
-            sx_fn = self.fn_ptr,
-        };
-    }
-}
+// User-facing surface: `xx my_closure : Block` for ANY closure
+// signature. No per-signature stdlib boilerplate. The shared
+// infrastructure above (`Block`, `BlockDescriptor`,
+// `_NSConcreteStackBlock`, `__sx_block_descriptor`) is referenced
+// by the synthesised code; users only need to `#import` this module.