refactor(ntx-builder): replace dataless Notify with per-account watch

[SantiagoPittella]

Closes #2259

Summary

Replaces the dataless Notify the ntx-builder coordinator used to wake account actors with a per-account watch::channel<AccountView> that carries the committed-block state to each actor directly.

Why:

• On every committed block the coordinator woke every active actor without carrying data, so each actor then re-checked "what happened" from SQLite.

How:

• The coordinator pushes a AccountView { chain_tip, last_committed_tx, notes_seen } per account via a watch channel. Fields are cumulative, so an actor that slept through several blocks reads one summarised view and answers everything in memory.
• Inclusion detection becomes just an in-memory compare and removes the per-block account_last_tx query.
• available_notes now also returns next_retry_block. Actors schedule a single re-check instead of polling each block.
• The idle-timeout shutdown race is handled by a biased select! + an authoritative account_has_pending_notes recheck on reap, replacing the old Notify-permit trick.

Changelog

[[entry]]
scope = "ntx-builder"
impact = "changed"
description = "Drive account actors from a per-account watch<AccountView> instead of a dataless Notify, eliminating per-block DB polling for landing checks and note availability."