feat(http): surface §7 Partial results on Prometheus warnings

[zzylol]

Summary

Task #34 gap #2 of 3. #48's dispatcher activated per-segment execution for combinable stats (Count/Sum/Min/Max) but refused to run for non-combinable stats (Quantile/Topk/Cardinality/Rate/Increase) — those still fell through to the single-agg path and saw the same data cliff the dispatcher was written to prevent. Reason: there was no way to tell the HTTP caller "this answer is Partial because the query spans a reconfigure boundary and the statistic can't be combined scalarly."

This PR adds the Partial surface.

What's in this PR

• QueryResult::vector_with_warnings + warnings() accessor. InstantVector / RangeVector gain a #[serde(default, skip_serializing_if = \"Vec::is_empty\")] warnings: Vec<String> field. Default constructors (QueryResult::vector / matrix) stay wire-compatible — they serialise without the field when empty, so every existing caller and snapshot test holds.
• Prometheus adapter: top-level warnings: [] on PrometheusResponse (also skip-if-empty), matching upstream's native API. format_success_response + format_range_success_response thread through any warnings the engine populated.
• Dispatcher: drops the combinable-only early return. The full loop runs for every statistic whenever the timeline has ≥2 segments. combine_statistic still returns Full(v) for cleanly combinable inputs and Partial { covered, missing } everywhere else — on Partial we accumulate the covered scalar (when present), flag any_partial, and build a human-readable warnings list carrying metric, range, statistic, dropped-group count, and up to three unresolved segments. Overflow summarised; full set still inspectable via GET /api/v1/db/timeline.

Example response shape

```json
{
"status": "success",
"data": { "resultType": "vector", "result": [...] },
"warnings": [
"partial result: query spans 2 schemas for metric 'latency' over [1000, 9000] and the requested statistic Quantile is not cleanly combinable across schema boundaries — see GET /api/v1/db/timeline?metric=latency&start_ms=1000&end_ms=9000 for the full segment map",
"segment agg_id=1 [1000, 5000) status=Retired coverage=Sketch unresolved"
]
}
```

Validation

• `cargo test -p query_engine_rust --lib` — 734 pass (was 728; +5 new + 1 that was previously filtered became visible)
• `cargo clippy --all-targets -- -D warnings` — clean
• `cargo fmt --all -- --check` — clean
• New tests: 3 in engines::query_result::tests (default-empty / with-warnings / matrix wire contract) + 2 in Prometheus adapter (response-side serialisation contract)

Follow-ups

• Integration test (task feat(sketch-db): Phase 5f-a — BackfillRegistry::coverage() classifier #34 gap feat: complete OTLP sketch ingest → SimpleMapStore #3) — seed two agg_ids for the same metric, query across the boundary with both a Sum and a Quantile, assert the response shape for each. Requires the full PrecomputeEngine setup harness; next PR.

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