mem: reject call-conv mismatches at bare-fn → fn-ptr coercion

Passing a default-conv sx function to a `callconv(.c)` fn-pointer slot
(e.g. pthread_create's start routine) used to silently mismatch ABIs:
the C-side caller didn't supply __sx_ctx, so the sx-side body read its
first user param as garbage. The bug surfaced as a SIGSEGV inside
ANativeWindow_setBuffersGeometry on Android during chess bringup.

Now the compiler rejects the coercion outright at the bare-fn name
lookup site:

  error: call-convention mismatch: 'sx_handler' is declared with
  default sx convention but the target type expects callconv(.c)

Also: `#foreign` declarations without an explicit `callconv` now default
to `.c` instead of `.default`. Every external C symbol is by definition
C-conv; the previous default silently typed `objc_msgSend` (et al.) as
default-conv, so the check would fire on the consumer side when the
user typed a fn-ptr as `callconv(.c)`. With the foreign-default fix,
the existing typed-msgSend casts in `std/objc.sx` and `gpu/metal.sx`
keep type-checking and the rule is "C-conv on both sides or neither."

Caught by the new check (fixed in the same commit):
- `ios_gl_proc` in `platform/uikit.sx` lacked callconv(.c) but was
  passed to `load_gl` whose `get_proc` slot expects it.
- `ffi_apply_callback` / `ffi_apply_callback2` in
  `examples/ffi-06-callback.sx` had default-conv fn-ptr params but
  the C bodies (in the companion .c) are unambiguously C-conv.

Regression test: `examples/131-callconv-mismatch-diagnostic.sx`
locks in the diagnostic shape (sx-conv fn → callconv(.c) slot).

153/153 example tests pass. Chess green on macOS / iOS sim / Android.

examples/131-callconv-mismatch-diagnostic.sx

@@ -0,0 +1,14 @@
+// Passing a default-conv sx function as a callconv(.c) fn-pointer
+// silently mismatches ABIs — historically that meant the C-side caller
+// supplied no `__sx_ctx` slot 0 and the sx-side body read garbage.
+// The compiler now rejects the coercion outright with a "call-convention
+// mismatch" diagnostic.
+
+#import "modules/std.sx";
+
+sx_handler :: (arg: *void) -> *void { return arg; }
+
+main :: () -> s32 {
+    fp : (*void) -> *void callconv(.c) = sx_handler;
+    return 0;
+}

examples/ffi-06-callback.sx

@@ -16,8 +16,8 @@
     #source "ffi-06-callback.c";
 };
 
-ffi_apply_callback  :: (cb: (s32) -> s32, value: s32) -> s32 #foreign;
-ffi_apply_callback2 :: (cb: (*void, s32) -> s32, ctx: *void, v: s32) -> s32 #foreign;
+ffi_apply_callback  :: (cb: (s32) -> s32 callconv(.c), value: s32) -> s32 #foreign;
+ffi_apply_callback2 :: (cb: (*void, s32) -> s32 callconv(.c), ctx: *void, v: s32) -> s32 #foreign;
 
 g_callback_hits : s32 = 0;
 g_callback_sum  : s32 = 0;