fix(desktop): cut steady-state relay traffic from polls and read-state echo

[tlongwell-block]

Problem

Desktop webview CPU stayed high after the presence-scope fix (#5830) and the shared useNow ticker (#5861). A per-kind byte tap hot-patched into relayClientSession.ts on a live desktop (~500 channels, large agent fleet; 850 s capture correlated with CPU sampling) showed the remaining steady-state relay traffic is mostly self-inflicted:

kind	what	share of inbound bytes	shape
30078	read-state	34%	our own ~44 KB nip44 blob echoed back every ~10-30 s while reading
30030	emoji union	33%	2-min poll refetching every member's full set (~300 KB burst)
30175	persona catalog	13%	same 2-min backstop pattern, ~150 KB per walk

CPU tracked the bursts directly: 3-5% in quiet 10 s buckets vs 44-54% in buckets containing a poll burst or read-state echo. (The kind-24200 observer-frame theory was tested and disproven by the same tap: 9.7% of bytes, steady trickle.)

Outcome

• Read-state echo drop. ReadStateManager remembers the ids of events it just published (FIFO set capped at 64) and drops their relay echoes before the nip44-decrypt + JSON.parse step. Ids are recorded before publishing so relay fan-out can't race the OK. The drop consumes the id, so a reconnect replay of the same event still parses normally. Events from other clients of the same pubkey are untouched.
• Poll backstops stretched 2 min → 20 min for the emoji union and persona catalog queries. The live subscriptions (invalidate on any new 30030/30175) and the reconnect invalidations remain the freshness paths; the poll only exists to cover a silently dropped live event. Behavior on publish, focus, and reconnect is unchanged.
• Mechanical: localStorage identity helpers moved to readStateIdentity.ts (no behavior change) to keep readStateManager.ts under the file-size ratchet.

Expected effect on the measured profile: the poll stretch cuts the 30030/30175 bursts (46% of inbound bytes) by 10x; the echo drop removes the recurring ~44 KB nip44-decrypt + parse per publish cycle (the echo still arrives on the wire — nostr filters cannot exclude own-author events — so this is a CPU/IPC saving, not a bandwidth one).

Acceptance

• New tests: echo dropped before decrypt (mutation-checked: disabling the drop fails the test), replayed duplicate of the same id still parses, foreign-client events always parse, published-id set stays capped when publishes fail (never-echoed ids).
• Full desktop suite 4794/4794, tsc --noEmit clean, pnpm check (biome + ratchets) clean at head.

Not addressed (follow-ups)

• The 44 KB blob itself (one read-state event carries all ~500 channels; a delta or per-channel-shard format is a protocol change).
• Duplicate delivery of the same events on concurrent history- subscriptions (relay/client dedupe).
• Webview RSS of 12.5 GB observed on the same machine — retention hunt is separate work; shrinking the heap multiplies the value of this PR since the GC floor scales with live-heap size.