ffi 1.28 backfill: locked-in test for #objc_call(bool)

Closes the runtime-verification gap from cluster 1.28: chess startup
doesn't reach the keyboard `becomeFirstResponder` / `resignFirstResponder`
path, so `#objc_call(bool)` was only compile-verified. This example
installs two BOOL-returning IMPs via `class_addMethod` (type encoding
"B@:") and dispatches both through `#objc_call(bool)`. Also exercises
the nil-receiver guarantee (libobjc returns a zero slot, which decodes
as false).

This is a test-add commit (per the FFI cadence rule): it locks in
current behavior without changing any lowering. Lowering shape is
identical to `#objc_call(u8)` at the ABI layer; this test makes the
source-level type explicit and gives `git bisect` a target if a
future emit_llvm change inadvertently breaks single-byte returns.

110/110 host tests pass.

tests/expected/ffi-objc-call-11-bool-return.exit

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

tests/expected/ffi-objc-call-11-bool-return.txt

@@ -0,0 +1,3 @@
+nil bool     = false
+yes = true
+no  = false