refactor(query-routing): centralize ASAP-first routing in EngineRouter; add range support (supersedes #309)

[zzylol]

What

Centralizes query routing in EngineRouter and makes http.rs a thin transport layer. Both instant and range queries now try the ASAP sketch tier first and fall back to the archive (thanos_query) on CapabilityMiss; accuracy == Exact goes straight to the archive.

Supersedes #309 — same Thanos /api/v1/query_range forwarding, but the engine selection lives in the router instead of being inlined in http.rs.

Why

Routing was split across two places: instant went through EngineRouter + compatible_storage_backends, while #309 added range routing inline in http.rs (try ASAP → on miss, hardcoded engine_by_id(thanos) lookup), bypassing the router entirely (which had no execute_range). That left http.rs making engine-selection decisions and the two query types routing by different criteria. This consolidates the decision into one place.

Changes

• query_engine_routing.rs: QueryEngine::execute_range default (→ CapabilityMiss); EngineRouter::execute_range mirroring execute()'s failover over the shared compatible_storage_backends sequence.
• asap_query_engine/engine.rs: ASAPQueryEngine::execute_range delegates to the inherent execute_range_promql_modern (so the router reaches the real warm-tier range path).
• thanos_query_engine/forward.rs: ThanosQueryEngine::execute_range forwards to /api/v1/query_range (4xx → CapabilityMiss, 5xx/timeout → Backend), ported from feat (thanos-ASAPCollector): implement /api/v1/query_range forwarding in ThanosQueryEngine #309.
• capability_matching.rs: compatible_storage_backends → ASAP-first (Approximate → [SketchStore, GorillaObjectStore]), Exact → [GorillaObjectStore], PrometheusRemote unchanged; tests updated.
• http.rs: process_range_query_request thinned to a single query_router.execute_range(...); inline Thanos lookup removed.

Notes

• The Exact → archive gate is wired but dormant: the live query path still hardcodes accuracy = Approximate (matches the existing instant path). Deriving Exact from a request (query param / header / per-metric config) is a follow-up — until then everything is ASAP-first.

Verification

cargo check -p data_plane + the routing / thanos / http range / capability_matching test suites all green (incl. the warm-tier range e2e). Verified in an isolated build env (asap_sketchlib main + ASAPCollector main); CI will compile against its own pinned versions.