feat: Add subscribe_resolved (latest entry per key + blob-complete gating)

[cbenhagen]

Description

Adds a subscription that correlates live sync events with Query::single_latest_per_key() (latest entry per key, timestamp/hash tie-break), waits until that entry’s blob is locally complete (or the entry is empty), then emits the resolved key + entry (optional bytes). This closes the gap between per-author InsertLocal / InsertRemote and ContentReady { hash }, which carries no key—the store has no hash→key index, so apps otherwise re-query or track pending state themselves.

Problem / baseline

• LiveEvent inserts are per-author; they are not necessarily the latest entry for that key.
• Latest-for-key is defined by Query::single_latest_per_key() and existing store selection rules.
• ContentReady { hash } does not name a key; mapping completion to keys is non-trivial without re-querying.

Behavior

When the latest entry for a key in scope may have changed, re-resolve via the store; emit only when the blob is Complete (or the entry is empty). Resolution always uses the query, not the raw insert event.

Implementation

• Dirty keys on scoped inserts; flush with get_one / get_many under single_latest_per_key.
• Blob status gating; pending key→hash; reconcile on ContentReady; drop stale pending if the winner changes.
• Blob reads use a small internal ResolveBlobs trait (production delegates to iroh-blobs); if get_bytes fails after status is Complete, the subscription stream yields Err and the resolver keeps running.
• Forward LiveEvent on a dedicated task into a bounded mpsc so slow resolution does not fill Engine::subscribe’s bounded channel.

API

• Module subscribe_resolved: subscribe_resolved_with, ResolvedFetcher, ResolvedKeyValue, ResolvedSubscribeOpts.
• Engine::subscribe_resolved, Doc::subscribe_resolved.
• Store helpers / docs for single_latest_per_key with key prefix vs author filter.

Tests

• Integration (tests/subscribe_resolved.rs, one case in tests/sync.rs): two authors / latest-per-key; sync across nodes + blob; burst inserts with slowed resolution; include_empty + delete; initial_snapshot; Query::author() after grouping; whole-replica scope (KeyFilter::Any / dirty_all flush).
• Unit (subscribe_resolved module): mock ResolveBlobs for blob status error then recovery, get_bytes error on the stream, and “no Ok while status keeps failing.”

Caveats

• Empty / tombstone-style winners: use .include_empty() on the scope Query, not ResolvedSubscribeOpts.
• Blob not Complete: no item for that key until complete or winner changes; get_bytes failures after Complete surface as stream Err.
• Inserts are not assumed to be the winner; always resolve through the store.

Out of scope

• Engine-native “resolved key” events in LiveActor (possible follow-up).
• New irpc for remote blob bytes (v1 assumes client-side blob API where needed).

Breaking Changes

None; additive API.

Notes & open questions

None.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.