Retry listen(0)/bind(0) when the preferred port is taken, gated by
BARE_OS_BIND_FALLBACK (default on; set 0/false for strict bind).
- Add lib/bare-os-bind-fallback.js and use it from bare-os-www-initd and
bare-openssh (including privilege-path + holesail actual port).
- POSIX socket bridge in index.js: TCP listen and UDP bind fallbacks.
- bare-holesail + bare-holesail-managed: retry Holesail ready(); persist
resolved listen port in managed state when it changes.
- Add test.bare-os-bind-fallback.js.
