refactor(ffi-linkage): Phase 6.3 — migrate std/ #foreign→extern

Pure source rename across 11 std modules (~60 sites): cli/core/fmt/fs/log/
net/kqueue/process/socket/thread/time/trace. All fn-decl markers — bare
'#foreign;', '#foreign libc;'/'#foreign tlib;' (LIB ref), and
'#foreign libc "csym";' (LIB+rename) → the same 'extern …' tail (extern carries
the identical [LIB] ["csym"] axis). Plus 2 stale comment mentions (fmt/fs).
No class forms in std. These modules ARE host-corpus-exercised, so the empty
snapshot diff is direct validation. Suite green (647 corpus / 444 unit, 0
failed).

library/modules/std/fmt.sx

@@ -177,7 +177,7 @@ substr :: (s: string, start: i64, len: i64) -> string {
 // ── cstring: the C-boundary string ────────────────────────────────────
 // `cstring` is ONE pointer to a null-terminated u8 buffer — C's `char *`.
 // It carries no length (`cstring_len` walks to the terminator) and
-// crosses `#foreign` boundaries verbatim in both directions; `?cstring`
+// crosses `extern` boundaries verbatim in both directions; `?cstring`
 // is the nullable case (null pointer = absent). String LITERALS coerce
 // to `cstring` implicitly — their bytes are terminated constants; every
 // other `string` must materialize through `to_cstring`.