perf(usage): keep per-source usage caches off the main thread (#26980)

* perf(usage): keep per-source usage caches off the main thread

The usage stores kept every provider's per-source scan cache
(processedFiles / processedDatabases) inside the one JSON file main
loads at startup. On a large Claude history that file is 56 MB, 43 MB
of it per-source records only a scan reads, so main paid a sync
read+parse at launch, a structured clone of the whole cache to and
from the scan worker on every scan, and a pretty-printed rewrite
after every scan.

The usage scan worker now owns those records in a sidecar
(`<cache>-sources.json`): it reads it before a scan, writes it after
(compact, durable rename), and returns only sessions and daily
aggregates. Main keeps and writes only that report, compact. A cache
still carrying inline records is split on the worker the first time
it is loaded (above 8 MB); smaller ones parse on main and rescan cold.
Sync IPC getters and automation lookups wait for the load.

Synthetic 56 MB cache, same machine, interleaved runs:
- first launch: 108 ms sync parse -> 18 ms main busy, 4 ms longest block
- later launches: ~69 ms -> ~7 ms
- forced scan, longest main-thread block: 89 ms -> 11 ms
  (real built app, headless: 84-94 ms -> 11-13 ms)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(usage): preserve cache attribution and legacy warm scans

* fix(usage): reclaim cache temps after worker retirement

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Neil <neil@stably.ai>

src/main/opencode-usage/opencode-usage-provider.ts

@@ -1,10 +1,6 @@
 import type { UsageProvider } from '../usage/usage-provider-contract'
 import { scanOpenCodeUsageDatabasesViaWorker } from '../usage/usage-scan-worker-spawn'
-import type {
-  OpenCodeUsageDailyAggregate,
-  OpenCodeUsagePersistedDatabase,
-  OpenCodeUsageSession
-} from './types'
+import type { OpenCodeUsageDailyAggregate, OpenCodeUsageSession } from './types'
 
 // Why: v4 reads OpenCode 2's `session_v2` table; v3 caches miss every v2 session.
 // v5 merges a migrated session's two rows per column instead of picking one, so
@@ -17,9 +13,4 @@ export const openCodeUsageProvider = {
   label: 'OpenCode',
   schemaVersion: OPENCODE_USAGE_SCHEMA_VERSION,
   scan: scanOpenCodeUsageDatabasesViaWorker
-} satisfies UsageProvider<
-  'processedDatabases',
-  OpenCodeUsagePersistedDatabase,
-  OpenCodeUsageSession,
-  OpenCodeUsageDailyAggregate
->
+} satisfies UsageProvider<OpenCodeUsageSession, OpenCodeUsageDailyAggregate>