fix(portability): make test green on macOS, GNU-only idioms replaced

The suite and seven libs assumed a GNU userland: `cmd | grep -q` under
pipefail (grep exits at the first match, the producer takes SIGPIPE,
rc 141 → 15 false "merged into" FAILs in gitflow-test), `sed -i` with no
suffix, `wc -l` padding compared as a string, `stat -c`, `touch -d`,
`realpath -m` (gstack-links refusal never fired), bare `timeout` off the
sanitized PATH (design gate READY BUT UNVERIFIED), `grep -oP` (update-all
emptied the plugin list). Thirteen suites were red on this machine.

Portable forms on the native userland of both OS: producer captured out
of the pipeline, `sed -i.bak` in tests and a temp-sibling `sed_profile`
on the user dotfile, `tr -d ' '`, python3 perms, `touch -t`, a
`realpath -m` emulation that refuses `..`, perl `alarm` for the 15 s
bound, `sed -n` token extraction. Regression tests: gstack-links T4b,
doctrine-citers Alphabet/Alpha flip, profile-set-managed T18 (failing CLI
no longer aborts `set`), new portability-census suite over the tracked
shell files. Linux run deferred (see TODO).

lib/doc-shape.sh

@@ -67,7 +67,8 @@ _path_exceeds_reason() {
     printf 'new/untracked doc (a creation, not a MINOR drift-patch): %s\n' "$p"
     return
   fi
-  if git diff HEAD -- "$p" | grep -Eq '^\+#{1,6}[ \t]'; then
+  # producer out of the pipe: grep -q would SIGPIPE git (fails open on macOS)
+  if grep -Eq '^\+#{1,6}[ \t]' < <(git diff HEAD -- "$p"); then
     printf 'adds a section heading (structural change, not a factual tweak): %s\n' "$p"
     return
   fi