feat(metrics): wire §6.3 barrier counter into all OTLP ingest paths - #51

Captures the post-PR-#51 punch list for sketchDB:
- 5 blockers for paper submission (cold S3 fallback,
  accuracy-profile library, e2e capability-miss test,
  format-versioning tests, correctness proofs)
- 4 future-work items (Phase 4 compaction, Tier-2 read cache,
  cross-sketch-type combine, ingest-path error counter parity)

Accuracy-profile library references sketchlib-bench's DESIGN.md
for the theoretical derivations.

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

Addresses TODO.md blocker #1. Unblocks the paper's "sketches for
hot, exact for cold" story: capability-miss queries now serve
exact answers from raw observability samples in the cold tier
instead of hitting Prometheus (or failing) unconditionally.

## What's new

* `drivers/query/fallback/cold_store/` — storage-agnostic
  `ColdStore` trait + `LocalFsColdStore` impl. On-disk layout
  (`raw/<metric>/YYYY/MM/DD/HH/part-NNNNNN.jsonl`) is identical
  to what a future S3 cold store will use, so the adapter stays
  source-compatible when we swap backends.
* `drivers/query/fallback/s3_adapter.rs` — `ColdFallback<S>`
  implements `FallbackClient`. Parses PromQL, extracts
  `(metric, predicates, op)`, scans the cold store, computes
  the answer. Supported shapes for v1:
  - bare instant vector selector (`metric{labels}`)
  - no-grouping scalar aggregation
    (`sum|count|avg|min|max(metric{...})`)
  Label matchers: `=`, `!=` — regex delegated upstream.
  Anything outside this surface falls through to the optional
  inner `FallbackClient` (chain-of-responsibility, typically
  the existing Prometheus proxy).
* `drivers/query/fallback/metrics.rs` — hot/cold telemetry
  counters (`queryengine_hot_queries_total`,
  `queryengine_cold_queries_total`,
  `queryengine_cold_bytes_served_total`), keyed by
  `(metric, shape)`. Mirrors the PR #51
  schema-barrier-counter pattern.
* `AdapterConfig::prometheus_promql_with_cold` convenience
  constructor that composes the cold adapter in front of a
  Prometheus proxy.

## Routing

No engine changes needed. The existing `process_query_request`
already falls through to `FallbackClient` on engine-miss —
configuring the fallback as a `ColdFallback` naturally routes
Purged-segment / capability-miss queries through the cold tier
with the Prometheus proxy as the tail-of-chain for unsupported
shapes.

## Tests

752 → 777 tests (+25):

* 5 unit tests on the JSONL format + hour-prefix helpers
* 4 on `LocalFsColdStore::scan` (range filter, missing prefix,
  hour-boundary span)
* 10 on `ColdFallback` plan extraction + aggregation + float
  formatting
* 6 end-to-end HTTP integration tests in
  `tests/cold_fallback_tests.rs` covering: bare selector, sum
  aggregation, label filtering, unsupported-shape delegation,
  telemetry-counter increments, and the "Purged range served
  from raw" paper story.