feat(sketch_db): sid-level timeline_for_metric - #183

Schema retirement #2 of 5. The `/api/v1/db/timeline` handler now
reads from `state.sketch_index` (always attached) via
`sketch_db::query::timeline::timeline_for_metric` (PR #183) instead
of `state.schemas.timeline_for_metric`.

User-visible behavior:

- Endpoint always works (no more 503 when schema-registry isn't
  wired — the sid catalog is always present).
- `agg_id` field carries a stable content-derived signature id
  (xxh64 of `metric + agg_kind + group_by_keys`) instead of the
  controller-emitted `agg_id` (which is gone after M2.2 / PR #152).
- Segments now reflect the sid catalog directly; reconfigure
  semantics propagate once the next sub-PR (lifecycle reconcile)
  lands.

Test impact:

- `test_get_timeline_without_registry_returns_503` rewritten to
  `..._with_no_sids_returns_empty_200` matching the new semantics.
- `test_get_timeline_returns_segments_after_reconfigure` ignored
  (uses POST /streaming-config → SchemaRegistry::reconcile, which
  doesn't yet propagate to the sid catalog; re-enabled in the next
  sub-PR with the lifecycle reconcile).

789/792 lib tests pass (3 ignored — 2 pre-existing + this one).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

#185)

Schema retirement #3. Repoint `ASAPQueryEngine::timeline_for_query`
from `SchemaRegistry::timeline_for_metric` to the sid-level
`storage_engines::sketch_db::query::timeline::timeline_for_metric`
landed in #183. The cross-reconfigure dispatcher
(`try_handle_query_promql_via_timeline`) now reads its segments from
the sid catalog rather than from `SchemaRegistry`.

The sid-level timeline populates `TimelineSegment.agg_id` with a
content-hash of `(metric, agg_kind, group_by_keys)` rather than a
`StreamingConfig.aggregation_id`. Until schema retirement #5 ports
the per-segment dispatch to sid-level evaluation, the
segment-→-aggregation_config lookup inside the dispatcher is
best-effort: when no segment resolves to an in-config aggregation
the dispatcher returns `None` so the caller falls back to the
default single-agg path instead of regressing cross-reconfigure
queries to empty-result-plus-warnings.

The schema retirement plan keeps the `schema_registry` field on
`ASAPQueryEngine` alive for now — it's still referenced by the
ingest barrier and the swap-handler driver. Both go away in
retirements #4 + #5.

Two tests in `tests/schema_timeline_dispatch_tests.rs` are marked
`#[ignore]`: they build two distinct `AggregationConfig`s with
identical content (same metric / Sum / `host` grouping). In the
sid catalog those collapse to one signature group → one segment,
so the dispatcher can no longer reproduce the
schema-boundary-stitch scenario from a SchemaRegistry-shaped
fixture. The third single-schema regression test still passes
unchanged. Re-enabling these is part of retirement #5 (sid-level
dispatch) or a fixture rewrite that uses two genuinely distinct
signatures.

787/787 lib tests pass; 5 ignored.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>