feat(dev-server): support `poll` for filesystem polling watch mode

[lukekania]

Goal

Honor the poll option (interval in ms) so projects on filesystems where notify-style events don't work reliably (NFS, some Docker bind-mounts, WSL crossing the Windows/Linux boundary) can fall back to polling.

Current state

poll is silently dropped. Workaround: nothing — affected users can't use ngc-rs serve.

Scope

• packages/builder/schemas/dev-server.json: add poll: { type: number }.
• packages/builder/src/serve/options.ts: forward as --poll <ms>.
• crates/cli/src/serve_cmd.rs: accept --poll flag.
• crates/watch/: notify already supports a PollWatcher backend — switch to it when poll is set.

Definition of done

• ng serve --poll=2000 works on a Docker bind-mount where notify events don't fire.