ffi 0.8: #foreign call sites inside struct/protocol/closure/inline-if

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.

tests/expected/ffi-08-foreign-in-method.exit

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

tests/expected/ffi-08-foreign-in-method.txt

@@ -0,0 +1,5 @@
+method next 1 = 10
+method next 2 = 20
+protocol     = 60
+closure(5)   = 150
+inline if macos = 70