fix(portability): stop assuming GNU coreutils flags on macOS

BSD userland rejects or silently ignores several GNU spellings the repo used:

- `timeout` is not in a stock macOS at all (Homebrew installs it, and also as
  `gtimeout`). Without it every gates.sh check exited 127 and was recorded
  NOT-MET whatever the check actually did — a systematic false negative.
  The binary is now resolved once, with a pure-bash deadline behind it so the
  124 contract still holds where neither binary exists. GATES_TIMEOUT_BIN is
  overridable with `-` not `:-`, so an empty value forces that fallback: the
  suite passes 64/64 both ways, timeout cases included.
- `touch -d '10 days ago'` is GNU-only; perl's utime is the one spelling both
  platforms ship.
- BSD `wc -l` pads its count with leading spaces, so string compares failed as
  got[      48] want[48].
- `sed -i` needs a suffix argument on BSD AND does not expand \n in the
  replacement, so the release-candidate CHANGELOG edit silently did nothing
  and the assertion failed for the wrong reason. Rewritten in awk; the RC_TAG=0
  mode still REDs on the absent tag, so the test keeps its teeth.
- `/bin/grep` does not exist on macOS (grep lives in /usr/bin), and `stat -c`
  is GNU-only.

fast-libs 11/11, seo-data 221/221, release-candidate 5 GREEN / 0 RED.

lib/tests/run-release-candidate.sh

@@ -38,7 +38,11 @@ echo
 ( cd "$WORK" || exit 1
   bash "$GITFLOW" start release 4.0.0 >/dev/null            # base develop → release/4.0.0 (lib L49/L71)
   printf '4.0.0\n' > version.txt                            # prep: version bump
-  sed -i 's/## \[Unreleased\]/## [Unreleased]\n\n## [4.0.0] — 2026-06-30/' CHANGELOG.md
+  # awk, not `sed -i`: BSD sed (macOS) needs a suffix argument after -i AND
+  # does not expand \n in the replacement — the edit silently did nothing
+  # there, so the CHANGELOG assertion below failed for the wrong reason.
+  awk '{ print; if ($0 == "## [Unreleased]") { print ""; print "## [4.0.0] — 2026-06-30" } }' \
+    CHANGELOG.md > CHANGELOG.tmp && cat CHANGELOG.tmp > CHANGELOG.md && rm -f CHANGELOG.tmp
   git commit -qam "chore(release): 4.0.0 — version.txt + CHANGELOG"
   bash "$GITFLOW" finish >/dev/null                         # fan-out main+develop+delete (lib L108-111)
   # TAG = the gap. Lives in the SKILL (lib untouched). RED skips it, GREEN does it.