95/95 regression tests pass (+ffi-08-foreign-in-method).
One trivial C helper (`ffi_method_helper`) called from each of the
major sx surface constructs that can host an FFI site:
1. struct method body Counter.next
2. protocol impl method body impl Doubler for Counter
3. closure value body make_adder's `closure(...)`
4. comptime-gated branch `inline if OS == .macos { ... }`
No new ABI shapes — the lowering route a `#foreign` call site takes
shouldn't depend on its enclosing construct, and the test pins that
lemma. A future lowering refactor that, say, breaks protocol-dispatch
fast-paths for FFI-calling impl methods will fail here directly
instead of being caught only by the chess Android regression.
The `inline if` branches for ios/linux compile down to nothing on
macOS, so only the macOS arm fires at runtime — useful smoke test
that the comptime gate works around FFI sites too.
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