feat(watch): pure-shell polling fallback when inotifywait/fswatch are missing

[Chemaclass]

Why

--watch hard-fails when neither inotifywait nor fswatch is installed (src/watch.sh:33 prints an install hint and exits). On CI containers, fresh macOS machines, and minimal images that's a dead end, while a portable polling fallback is cheap to provide. Watchers in most test frameworks degrade to polling for exactly this reason.

Proposal

Add a pure-shell polling backend as the third branch of the backend detection (src/watch.sh:12-15):

1. Change detection without GNU/BSD stat divergence: use a sentinel file + find:
   • touch "$sentinel" at loop start,
   • sleep BASHUNIT_WATCH_INTERVAL (new env, default 2 seconds),
   • find <watched paths> -name '*.sh' -newer "$sentinel" -print (mirror whatever file set the inotify/fswatch branches watch — check src/watch.sh:67+ for the current event filters) — non-empty output means rerun.
     This avoids parsing stat (whose flags differ between macOS -f %m and GNU -c %Y) and works on Bash 3.0.
2. Print a one-line notice when falling back: polling mode + interval + hint that installing fswatch/inotifywait gives instant triggers. Keep the current install-hint text as part of that notice instead of an error.
3. Sentinel lives under the existing temp-dir helpers (temp_dir) and is cleaned on exit (reuse watch mode's existing trap/cleanup if present — read the full file first).

Edge cases

• First iteration must run the suite immediately (current behavior), then start polling.
• Deleted files don't show up via -newer; acceptable for a fallback — note it in docs.
• BASHUNIT_WATCH_INTERVAL validation: positive integer, else default with a warning (follow how other numeric envs are validated in src/env.sh).

Tests first (TDD)

Watch mode loops forever, so test the pieces: unit-test the new "detect changes since sentinel" function (touch sentinel, touch a file, assert detection; no changes, assert none). For the backend selection, unit-test that the poller is chosen when both commands are missing (mock bashunit::watch::_command_exists). Follow existing patterns in tests/unit/watch_test.sh if present — grep first.

Constraints

• Bash 3.0+; find -newer is POSIX.
• No behavior change when inotifywait/fswatch exist.
• Docs page for watch mode + CHANGELOG.md entry.

Acceptance criteria

• ./bashunit tests/ and ./bashunit --parallel tests/ pass.
• make sa, make lint pass; shfmt -w . produces no diff.
• Manual check documented in PR: rename fswatch away, run --watch, edit a test file, suite reruns within the interval.

Line references valid at commit 1ae3e82.

[Chemaclass]

Done in #793 (merged).