feat: add scalable MessagesSubscribe mesh invalidation stream

[LiranCohen]

Problem

Role-only invite/preauth nodes cannot open one broad role-authorized RecordsSubscribe stream. Production requires exact protocolPath and direct-parent context filters, which would grow to roughly 4 + members + 2*nodes subscriptions per node and O(N^2) server state.

Target design

Use one network-context-scoped MessagesSubscribe stream per node as the authoritative invalidation feed:

• prefix filters for the mesh member, node, ACL, and relay subtrees
• owner-issued Messages/Read observer grant scoped to the network context
• structured ProgressToken resume, EOSE, gap repair, and terminal grant revocation using the shared subscription lifecycle from fix: align DWN subscription protocol #254
• events only invalidate/coalesce; the daemon continues serving its immutable last-good snapshot
• full reconciliation only on startup/fresh establishment, progress gap, ambiguous event, health recovery, and slow anti-entropy

Required work

1. Update the Enbox SDK/server authorization projection so contextIdPrefix/protocolPathPrefix filters are contained by scoped Messages grants, with positive and negative tests.
2. Add a Go MessagesSubscribe builder/parser reusing the existing WebSocket reconnect state machine.
3. Issue, persist, renew, and revoke observer grants during node approval/removal; backfill existing nodes.
4. Expose stream health and switch healthy nodes from 30s polling to 5-10 minute anti-entropy.
5. Keep explicit polling fallback until observer grant coverage is confirmed.

This avoids non-atomic revision dual-writes and quadratic path-scoped subscriptions.