## Why
The rendered `<token_budget>` fragment identifies the thread and current
context window, but it does not expose enough lineage to identify the
first window in the thread or the immediately preceding window. Those
IDs also need to remain stable across compaction, resume, and rollback.
## What changed
- Track first, previous, and current UUIDv7 context-window IDs in
auto-compaction state.
- Render `thread_id`, `first_window_id`, `previous_window_id`, and the
current window ID in the full `<token_budget>` fragment.
- Persist the first and previous window IDs in compacted rollout
checkpoints and restore them during rollout reconstruction.
- Preserve compatibility with older compacted records that do not
contain the new optional fields.
- Update focused state, rendering, reconstruction, rollback, and
serialization coverage.
## Validation
- `just test -p codex-core token_budget`
- `just test -p codex-protocol compacted_item::tests`
- `just test -p codex-core tracks_prefill_and_window_boundaries`
- `just test -p codex-core
reconstruct_history_uses_replacement_history_verbatim`
- `just test -p codex-core
thread_rollback_restores_cleared_reference_context_item_after_compaction`
## Why
The token-budget context currently identifies a context window by its
thread-local sequence number. A UUIDv7 gives the model a stable opaque
identity that remains fixed for a window and rotates when compaction or
`new_context` starts the next one.
## What changed
- Preserve the existing monotonic value as `window_number` and add a
UUIDv7 `window_id` to `CompactedItem`.
- Generate and rotate the UUID with auto-compaction window state,
persist it alongside the number, and reconstruct it on resume and
rollback.
- Accept legacy compacted rollout records where the numeric `window_id`
represented the window number.
- Use the UUID only in token-budget context; existing request headers
and metadata continue using `thread_id:window_number`.
## Testing
- `just test -p codex-protocol compacted_item::tests`
- `just test -p codex-core token_budget`
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