The session-store cache used only mtime for invalidation. In fast CI
runs (especially under bun), test writes to the session store can
complete within the same filesystem mtime granularity (~1s on HFS+/ext4),
so the cache returns stale data. This caused non-deterministic failures
in model precedence tests where a session override written to disk was
not observed by the next loadSessionStore() call.
Fix: add file size as a secondary cache invalidation signal. The cache
now checks both mtimeMs and sizeBytes — if either differs from the
cached values, it reloads from disk.
Changes:
- cache-utils.ts: add getFileSizeBytes() helper
- sessions/store.ts: extend SessionStoreCacheEntry with sizeBytes field,
check size in cache-hit path, populate size on cache writes
- sessions.cache.test.ts: add regression test for same-mtime rewrite
