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.

tests/expected/131-callconv-mismatch-diagnostic.exit

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

tests/expected/131-callconv-mismatch-diagnostic.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/131-callconv-mismatch-diagnostic.sx:12:42: error: call-convention mismatch: 'sx_handler' is declared with default sx convention but the target type expects callconv(.c)

tests/expected/ffi-jni-call-03-methodid-sharing.ir

@@ -334,5 +334,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-04-jint-return.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-05-jlong-return.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-06-jdouble-return.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-07-jboolean-return.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-08-jobject-return.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-call-09-static.ir

@@ -306,5 +306,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-class-08-call.ir

@@ -309,5 +309,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-jni-env-02-lexical-direct.ir

@@ -307,5 +307,3 @@ declare ptr @sx_jni_env_tl_get() #0
 declare void @sx_jni_env_tl_set(ptr) #0
 
 declare i64 @write(i32, ptr, i64)
-
-

tests/expected/ffi-objc-call-03-selector-sharing.ir

@@ -390,5 +390,3 @@ entry:
   store ptr %selN, ptr @OBJC_SELECTOR_REFERENCES_release, align 8
   ret void
 }
-
-

tests/expected/ffi-objc-call-06-sret-return.ir

@@ -3281,5 +3281,3 @@ entry:
   store ptr %sel, ptr @OBJC_SELECTOR_REFERENCES_tripleValue, align 8
   ret void
 }
-
-