feat(presence): PRES-3a — whiteboard cursors on the real transport

[th3-br41n]

Builds on PRES-2b (now on main). The whiteboard already renders remote cursors + selection from awareness — it just ran on a local no-transport channel (single-device). This swaps it onto the real DEK-sealed presence transport, so two users on the same board see each other's cursors, with zero change to the publisher / peer derivation / overlay paint (the transport abstraction's whole point).

Changes

• presence-channel.ts — presenceAwarenessFor(boardEntityId): the live transport in the shell (services.presence.publish/untrack + presence.onPeers, keyed on the board's entity id), else a local single-device channel (standalone / preview).
• engine.ts — awareness is now re-bindable; bindPresence(boardId) on every openBoard swaps the transport to the open board (boards switch in-window; the real id only lands on open), tearing down the prior board's presence (final clear + untrack). Closures read the current binding at call time; only the change listener re-attaches.
• storage/runtime.ts — consumer typing for services.presence + presence.onPeers.

Verification

• 8 presence-channel tests (+2: shell publishes to the service · inbound peer push → remote state · untrack on destroy · standalone → local).
• 239 whiteboard-logic tests green; whiteboard build clean (runtime imports survive the type-strip); full typecheck (packages + 12 apps) + lint clean.
• Behavioural (two cursors actually appear across two shells) is the PRES-4 /pentester two-shell dogfood — now unblocked by this.

Remaining PRES-3

The fleet header <PresenceStack> avatar mount across the React shared-entity apps (Notes/Database/…) — likely with an SDK extraction of the transport glue (rule-of-three). Whiteboard is the canvas-cursor reference; the header stacks are the who's-here-avatars half.

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