refactor(ffi-linkage): Phase 9.3 — purge 'foreign' from comments (src caps + examples + docs)

src/: ~21 capital-Foreign comments the case-sensitive verify grep missed
(Foreign-class→Runtime-class, Foreign path→Runtime path, Foreign decls→Extern decls,
FOREIGN function→extern function) across calls/inst/ffi_objc/jni_descriptor/emit_llvm/
c_import/lower.*/ops. src 'foreign' now = ONLY the hash_foreign token + 4 rejection
messages (9.0-delete targets). examples/*.sx comments → extern/runtime-class (1219
stdout regen; KEPT 1176). docs/inline-asm-design + debugger purged. Comments only —
no build impact. 9.0 ratified: DELETE hash_foreign token next.

examples/1218-ffi-foreign-cvariadic.sx

@@ -1,4 +1,4 @@
-// `#foreign` C-variadic tail: trailing `..args: []T` on a foreign fn maps
+// `extern` C-variadic tail: trailing `..args: []T` on a extern fn maps
 // to the C calling convention's `...`. Extras at the call site are
 // passed via the variadic slot with the standard default argument
 // promotion (i8/i16/bool → i32, f32 → f64) applied implicitly.