mvp v7: dual-routing + freshness pattern registration

[zzylol]

Summary

Closes the v6.1 architectural gap that forced issue #46 criteria ④ (warm-tier accuracy) and ⑤ (cold-fallback) to be exclusive — under v6.1 every metric had to pick one engine, so quantile-on-warm and count(...{service=\"payments\"})-on-archive could not both succeed for the same metric.

Two backend changes land together:

(A) Dual-routing per metric

• BackendStorageRouting now holds Vec<RoutingTarget> per metric, with an optional applies_to_query_shape: [count, topk, rate_post_hoc, quantile, sum, last_over_time, other] filter on each target.
• New QueryShape taxonomy + classify_query_shape(parsed_ast) extracts a coarse-grained shape from the parsed PromQL.
• lookup_with_shape(metric, shape) walks targets in two passes: explicit shape-match first, then default-slot fallback.
• Backwards-compatible YAML loader — existing single-target metrics: map entries decode to single-element target lists; new routes: list lets one metric fan out to multiple targets. A metric in BOTH wins from routes:.
• http.rs::resolve_metric_storage now classifies the parsed AST and passes the shape to lookup_with_shape, so http_requests_total can serve quantile_over_time(...) from the warm tier AND count(...)/topk(...)/rate(...) from the cold archive.

(B) Freshness pattern registration (GorillaQueryEngine)

• Add LastOverTime to QueryStatistic + AdditiveOp::Last on the streaming-additive accumulator. The existing AdditiveAccumulator already tracks (last_ts, last_value); the finaliser just returns last_value.
• Planner accepts last_over_time(<metric>[<range>]) and routes it through the streaming-additive path.
• Issue chore(tls): reqwest rustls-tls — drop libssl, make Dockerfile base-agnostic #46 ⑥ freshness probes encode unix_ts_ms_of_emission in the cumulative counter value; this gives the replay client a usable observed_value to subtract.

(B) is option (b) from the v7 spec — hard-coded support in the existing engine — chosen because it's <100 LOC and the existing AdditiveAccumulator already tracks the right state.

Tests

• 11 new routing-table unit tests (single-target preserved, multi-target shape selection, mixed metrics: + routes:, classify_query_shape over the canonical v6 demo PromQL shapes).
• 2 production-path HTTP integration tests (http_v7_dual_routing_count_lands_on_archive, http_v7_dual_routing_quantile_stays_on_warm_tier) mirror the existing http_routes_archive_metric_to_gorilla_engine test.
• 2 GorillaQueryEngine tests for last_over_time (basic + unordered-samples-pick-largest-ts).
• Pre-existing 34 backend test failures unchanged.

Test plan

• cargo build -p query_engine_rust — passes
• cargo test -p query_engine_rust --lib backend_storage_routing — 20/20 pass
• cargo test -p query_engine_rust --lib http_v7_dual — 2/2 pass
• cargo test -p query_engine_rust --lib gorilla_engine — 27/27 pass (incl. 2 new last_over_time)
• Full backend lib tests: 873 pass / 34 pre-existing fail / 9 ignored (vs origin/main: 854 pass / 34 fail / 9 ignored — +19 new passing tests, 0 new failures)
• v7 demo rerun on collector side (separate PR) — landing this PR first because path-dep

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