Desktop: auto-compaction writes to two transcripts in parallel, then the session switches files mid-conversation (history appears to roll back)

## Summary

On Claude Desktop, after auto-compaction the session's visible conversation history was
abruptly replaced mid-conversation — it appeared to "roll back" to a much earlier state.
No data was lost on disk, but the session became pinned to the compacted transcript,
hiding ~76 turns of detailed history.

Inspecting the transcripts shows the old and new transcript files were **both being written
to in parallel for ~43 minutes** before the switch.

## Environment

- Claude Desktop 1.24012.9
- macOS 26.5.2 (arm64)
- Model: claude-opus-5, effort: high
- Session was long-running (created ~26h earlier), multiple compactions had occurred

## What happened

Timeline for a single desktop session (`local_a1f2813a-…`):

| Time (UTC) | Event |
|---|---|
| 00:18:21 | Compaction creates a new transcript `207cc5f9…`, copying 248 entries forward |
| 00:18 – 01:01 | **Both** `30de0e57…` (old) and `207cc5f9…` (new) receive writes |
| 01:01:42 | Last write to the old transcript (252 entries were appended after 00:18) |
| 01:01:52 onward | Only the new transcript continues |

From the UI, the conversation visibly changed at that point: the detailed history was
replaced by a summary plus a handful of recent turns.

## Evidence

- The two transcript files share **248 entries with identical UUIDs**, spanning
00:18:21 → 00:47:42 — i.e. the same conversation was recorded into both files, not two
independent conversations.
- The old transcript kept receiving new entries until 01:01:42, i.e. **43 minutes after**
the new transcript was created.
- The desktop session metadata's `cliSessionId` pointed at the compacted transcript
(504 lines / 9 user turns), while the fuller transcript (2389 entries / 76 user turns)
was still present on disk and readable via `claude --resume `.

## Impact

- Conversation history appears to be lost from the UI. It is recoverable, but only if the
user knows to look in `~/.claude/projects//*.jsonl` and resume the older
transcript id manually.
- The two transcripts diverge: work done after the switch exists only in the new file, so
restoring the fuller history means the session no longer "remembers" the most recent turns.

## Workaround

Edit the desktop session metadata under
`~/Library/Application Support/Claude/claude-code-sessions/<…>/local_.json`
and point `cliSessionId` at the older transcript, then fully quit and relaunch the app.
This restored the expected conversation.

## Related

- #66947 — same underlying area (compaction re-writing entries with identical UUIDs), but a
different symptom: that report is about duplicate bubbles rendered *within one* transcript
file. Here the entries are duplicated *across two* transcript files that are both live, and
the session then switches which one it treats as authoritative. That issue was closed as
stale/not-planned; this may be worth revisiting together.
- #27242 — "data preserved but UI inaccessible" after compaction. Same user-facing outcome
(history unreachable from the UI), different trigger.

## Expected

Compaction should be seamless: a single transcript should be authoritative at any moment,
and the visible conversation should not change retroactively while the session is in use.

Start by examining the session metadata under ~/Library/Application Support/Claude/claude-code-sessions/ and the transcript files under ~/.claude/projects//*.jsonl, then use claude --resume to compare the old and compacted transcript IDs. Reproduce auto-compaction and verify that only one transcript remains authoritative, the UI does not switch history mid-session, and all turns remain accessible.

Written by the indexing model from the issue text.