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9203bf60ee perf(desktop): coalesce read state localStorage persistence (#5591 )

Follow-on to #5453/#5454's localStorage work — found while investigating
app-slowness reports on a real profile.

## Problem

`ReadStateManager.persistLocalState()` serialized and rewrote **all
three** read-state localStorage blobs (`buzz.channel-read-state.v2`,
`.publishable.v1`, `.source-created-at.v1`) synchronously on every
context advance. On a real profile (1,643 contexts, ~450K chars across
the three blobs) this produced ~880KB of localStorage sqlite WAL growth
per 30 seconds at idle, with writes every ~5s — steady main-thread
serialization + sync IPC for no user-visible benefit. Observed WAL size
on the affected profile: 94–114MB.

## Fix

- Local persistence coalesced behind a **1s trailing-edge timer**: a
burst of N advances produces one `writeStoredReadState` (one write per
blob).
- Pending dirty state **flushes synchronously** on `pagehide`, hidden
`visibilitychange`, `destroy()`, and before each relay publish — disk is
current before any relay event goes out.
- Hydration still persists immediately. Publish debounce (5s), merge
logic, and blob formats unchanged (`DEBOUNCE_MS` renamed to
`PUBLISH_DEBOUNCE_MS` only).

## Accepted residual

A hard kill (SIGKILL/power loss — not webview teardown) inside the 1s
window loses ≤1s of local read-state advances; relay max-merge bounds
the effect to a message flickering back unread. On the record per
review.

## Validation

- `readStateManager.test.mjs`: fake-timer/mock-storage coverage —
exactly one 3-blob write per burst (zero before the timer fires),
hidden-flush cancels the timer and persists, hydrate persists
immediately, pre-publish flush. Suite 26/26.
- Push gate at the pushed commit: desktop check, typecheck, full desktop
unit suite 4,670/4,670.
- Independent adversarial FULL REVIEW: **APPROVE** at tree `371a02cf`
(commit metadata rewritten afterward for attribution; tree identical) —
all six `persistLocalState` call sites traced, lifecycle/leak checks
(StrictMode remount, pubkey change), no external readers of the blob
keys.

---------

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Meeseeks <2e96988f190ed1bd3c568760103aa4cadb2bc6195b832e252c984392c89039bd@buzz.block.builderlab.xyz>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

2026-08-11 16:41:35 -07:00