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.

examples/ffi-08-foreign-in-method.c

@@ -0,0 +1,3 @@
+#include "ffi-08-foreign-in-method.h"
+
+int ffi_method_helper(int x) { return x * 10; }

examples/ffi-08-foreign-in-method.h

@@ -0,0 +1 @@
+int ffi_method_helper(int x);

examples/ffi-08-foreign-in-method.sx

@@ -0,0 +1,71 @@
+// Phase 0 baseline (PLAN-FFI.md step 0.8): `#foreign` C call sites
+// embedded inside the major sx surface constructs. None of these
+// touch a new ABI shape — they only verify lowering routes the call
+// through identically regardless of the enclosing context:
+//
+//   1. struct method body                  (Counter.next)
+//   2. protocol impl method body           (impl Doubler for Counter)
+//   3. closure value body                  (closure { ... })
+//   4. comptime-gated branch               (inline if OS == ...)
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+#import c {
+    #include "ffi-08-foreign-in-method.h";
+    #source "ffi-08-foreign-in-method.c";
+};
+
+// ── 1. Struct method calling a #foreign fn ───────────────────────────
+Counter :: struct {
+    seed: s32 = 0;
+    next :: (self: *Counter) -> s32 {
+        v := ffi_method_helper(self.seed);
+        self.seed += 1;
+        v;
+    }
+}
+
+// ── 2. Protocol impl method calling a #foreign fn ────────────────────
+Doubler :: protocol {
+    doubled :: (self: *Self) -> s32;
+}
+
+impl Doubler for Counter {
+    doubled :: (self: *Counter) -> s32 {
+        ffi_method_helper(self.seed) * 2;
+    }
+}
+
+// ── 3. Closure body calling a #foreign fn ────────────────────────────
+make_adder :: (bias: s32) -> Closure(s32) -> s32 {
+    closure((x: s32) -> s32 => ffi_method_helper(x) + bias);
+}
+
+main :: () -> s32 {
+    c : Counter = .{ seed = 1 };
+
+    // 1. struct method
+    print("method next 1 = {}\n", c.next());
+    print("method next 2 = {}\n", c.next());
+
+    // 2. protocol method (still operating on the now-bumped Counter)
+    print("protocol     = {}\n", c.doubled());
+
+    // 3. closure
+    adder := make_adder(100);
+    print("closure(5)   = {}\n", adder(5));
+
+    // 4. inline if OS branch — only one platform's call actually emits
+    inline if OS == .macos {
+        print("inline if macos = {}\n", ffi_method_helper(7));
+    }
+    inline if OS == .ios {
+        print("inline if ios = {}\n", ffi_method_helper(7));
+    }
+    inline if OS == .linux {
+        print("inline if linux = {}\n", ffi_method_helper(7));
+    }
+
+    0;
+}

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