cherry serve --poll for Docker bind mounts

Docker bind mounts from Windows/macOS hosts deliver no inotify events, so `cherry serve` in a container correctly degrades to no-live-reload (the probe catches the blind watcher). file_system ships a stat-polling backend that works fine across bind mounts — proven end-to-end during 0.2.0 development via the `:fs_watcher_start` seam.

Add `cherry serve --poll [--poll-interval MS]` wiring `FileSystem` `backend: :fs_poll`, so container users get real live reload with one flag. The probe should still run (poll delivers events, so it passes). Docs: a note in the CLI reference and the serve guide about when to reach for it.

Start at the `cherry serve` CLI entry point and trace the existing watcher probe, including the `:fs_watcher_start` seam. Wire the `--poll` and `--poll-interval MS` options to the FileSystem `backend: :fs_poll`, confirm live reload works without disabling the probe, and update the CLI reference and serve guide.

Written by the indexing model from the issue text.