fix(desktop): batch live channel subscriptions and pace reconnect replay under relay rate limits

[joahg]

Fixes the rate limiting the dev-mode client has been hitting since the relay's per-pubkey admission control shipped (block#1917, ~2 days ago): 50 EVENT/REQ/COUNT frames per 5 s window (10/sec average) per (tenant, pubkey).

The client opened two REQs per member channel (channel events + mentions) — ~159 REQs on cold start for a 72-channel identity, ~71 for the dev identity. Cold start and every reconnect blew the budget; the reconnect replay then re-blasted all REQs with only 50 ms between batches, re-triggering the limiter and causing the repeated disconnects.

Changes

Batch live subscriptions (useLiveChannelUpdates.ts)

• Channel-event and mention subscriptions now use multi-channel "#h" filters, 25 channels per REQ (LIVE_SUBSCRIPTION_BATCH_SIZE). REQ count drops from 2N to 2·⌈N/25⌉ — 6 REQs instead of ~144 for 72 channels.
• The diff-based subscription managers now key by batch instead of channel; an unchanged channel set produces identical batch keys and zero churn.
• buildChannelMentionFilter / subscribeToChannelMentionEvents accept a channel-ID list.

Attribute h-less events client-side (resolveChannelIdByEventReference)

• Reactions (kind:7) and deletions (kind:5) carry only an e tag; the relay routes them to #h subscriptions by resolving the target's channel server-side (crates/buzz-core/src/filter.rs). With one channel per subscription the client could inject the known channel (withChannelTagFallback); with batched filters it can't, so the handler now resolves the referenced event in loaded timeline caches instead. Unresolved events are dropped — without a loaded cache the timeline merge was a no-op anyway, and these kinds never hit unread/notification paths.

Pace reconnect replay (relayReconnectReplay.ts)

• REPLAY_INTER_BATCH_DELAY_MS 50 ms → 1000 ms: 8 REQs/sec stays under the relay's 10/sec average with headroom for concurrent publishes.
• shouldPageReconnectReplay now accepts multi-channel #h filters so batched subscriptions keep paged history replay.

Validation

• pnpm typecheck clean
• pnpm test: 4055/4055 pass, including new tests for batchChannelIds, resolveChannelIdByEventReference, and multi-channel paging eligibility

Linear: AGNTOPS-418