`listSessions` previously called `countSessionMessages` per file, which
streamed the entire JSONL through `readline` to count unique
user/assistant UUIDs. For a project with N sessions averaging M bytes
each, every /resume open paid O(N · M) wall time before showing the
picker — by far the dominant cost once a project accumulated many
multi-MB sessions.

This change:

- Drops the per-file count from listSessions / findSessionsByTitle.
  `SessionListItem.messageCount` is now optional. Callers that need
  a count call the new public `SessionService.countSessionMessages
  (sessionId)` lazily — typically only when a SessionPreview panel is
  about to display the badge.
- Pools a single 64KB tail-read buffer across the per-file metadata
  reads in listSessions / findSessionsByTitle, mirroring the pattern
  in claude-code's `enrichLogs`. The two helpers in
  `sessionStorageUtils` (`readLastJsonStringFieldSync` and
  `readLastJsonStringFieldsSync`) accept an optional caller-owned
  scratch buffer; one-off callers (rename, single-session lookup) pass
  nothing and keep the original alloc behaviour.
- Updates SessionPicker's row metadata to omit the "N messages"
  segment when `messageCount` isn't available, keeping the visual
  layout intact.

Tests:
- Pin that listSessions does not populate messageCount (regression
  guard against silently re-introducing the per-file scan).
- Smoke test the buffer-pool plumbing — same caller-owned buffer
  handed to two reads of different file sizes returns both correct
  values without state bleed.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

Tightens the title-write / title-read invariant so the picker's
metadata read is bounded to a fixed 2 × 64KB per file, regardless
of session length, with no fallback that scales with file size.

Writer (`ChatRecordingService`):
  - New `bytesSinceTitleAnchor` counter and `TITLE_REANCHOR_BYTES`
    threshold (32 KB, half of LITE_READ_BUF_SIZE).
  - `appendRecord` now updates the counter and, once it crosses the
    threshold while a title is set, re-appends a fresh `custom_title`
    record to EOF (`reanchorTitle`). The recursive append routes
    back through the same tracking path, which sees the title
    record and resets the counter to zero.
  - Sessions that never set a title pay zero overhead — the early
    return in `updateTitleAnchorTracking` short-circuits.

Reader (`sessionStorageUtils`):
  - `readLastJsonStringFieldSync` / `readLastJsonStringFieldsSync`
    now read the file's last 64 KB, fall back to the first 64 KB
    on miss, and return `undefined` if neither contains the field.
    The previous Phase-2 streaming full-file scan (capped at 64 MB)
    is removed entirely.
  - The pooled scratch buffer (already optional) now backs both the
    tail and head reads — only one allocation per `listSessions`
    page even with the head fallback firing.
  - `MAX_FULL_SCAN_BYTES` constant deleted (unused).

The two changes are coupled: the reader's tighter bound only works
because the writer guarantees the title stays in tail. A title
buried mid-file is now intentionally `undefined` (the picker
falls back to `firstPrompt` for display) rather than triggering a
full-file scan that would freeze the UI on long agent transcripts.

Tests:
  - `chatRecordingService.customTitle.test.ts` — three scenarios
    pinning the re-anchor invariant: threshold trigger, no-spurious
    on no-title sessions, no-trigger on small write bursts.
  - `sessionStorageUtils.test.ts` — replaces Phase-2 tests with
    head-window fallback tests; pins the new "buried beyond both
    windows returns undefined" contract; updates buffer-pool reuse
    test to cover both tail and head reads with the same scratch.

E2E coverage (35/35 scenarios on a separate harness against real
fs): R1–R10 reader contract, M1–M4 multi-field, W1–W7 writer
re-anchor, L1–L6 listing latency + regression pins, S1 stress
(200 × 3 MB), C1 concurrency, I1–I5 writer/reader integration.
Measured 2.6× speedup on listSessions(50) over 50 × 4 MB
sessions vs the legacy `countSessionMessages` baseline; speedup
scales linearly with average file size.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

- countSessionMessages: valid id counts unique user/assistant uuids and
  ignores other types/malformed lines; invalid id short-circuits without
  filesystem access; ENOENT degrades to 0 instead of bubbling.
- readLastJsonStringFieldsSync: mirror the single-field variant's
  scratch-buffer reuse test across a tail-hit then head-fallback to
  catch any decode that ignores bytesRead.
- ChatRecordingService re-anchor: legacy resumed session (source
  undefined) must omit titleSource on threshold-triggered re-anchor,
  not silently reclassify as 'manual'.

Co-authored-by: Qwen-Coder <noreply@alibabacloud.com>

`String.length` undercounts the on-disk size of multi-byte payloads
(CJK, emoji are 1 UTF-16 unit but 3 UTF-8 bytes), so a session full
of CJK content could push >96KB of writes past the last anchor before
the 32KB threshold thinks it has — silently drifting the title past
the 64KB tail window the picker scans. Switch the byte counter to
`Buffer.byteLength(..., 'utf8')` for parity with `jsonl.writeLine`.

Also reset `bytesSinceTitleAnchor` to 0 in the reanchor catch:
without it, a failing reanchor pins the counter at the threshold
and turns a single transient I/O fault into a per-record retry
storm. One missed anchor is the right tradeoff — finalize() will
re-emit on the next lifecycle event.

Co-authored-by: Qwen-Coder <noreply@alibabacloud.com>

…ession-list-perf

# Conflicts:
#	packages/core/src/services/sessionService.ts

The session metadata reader uses two bounded 64KB windows (tail + head)
since the perf rework on this branch — never a full-file scan. Two
comments still described the prior behavior. Reported in MR review.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>