feat(capability): Phase 5 — QueryEngine trait + StorageBackend routing for Gorilla-S3

[zzylol]

Summary

• Adds the storage-backend axis to asap_types::capability_matching so queries against metrics configured for Gorilla-S3 dispatch to GorillaQueryEngine instead of the warm-tier SimpleEngine. Per docs/design-gorilla-s3-cold-engine.md §8.
• Introduces a small QueryEngine async trait so the new EngineRouter can hold Arc<dyn QueryEngine> rather than case-on-concrete; both SimpleEngine and GorillaQueryEngine get an additive impl QueryEngine that does NOT change existing methods.
• Wires StreamingConfig.storage_backend (with #[serde(default)]) so the controller can push per-metric backend pins; pre-Phase-5 configs decode unchanged.

Deliverables

#	Item	Where
1	QueryEngine trait + EngineCapabilities	asap-query-engine/src/engines/router.rs
2	StorageBackend enum (4 variants) + AccuracyTarget enum	asap-common/dependencies/rs/asap_types/src/capability_matching.rs
3	compatible_storage_backends(stat, accuracy, metric_storage_config) -> Vec<StorageBackend>	same file; ordered failover list
4	EngineRouter (HashMap by data_source_id, register / execute / failover)	asap-query-engine/src/engines/router.rs
5	16 new tests (6 capability + 2 streaming_config serde + 6 router + 1 agreement + 1 default-pin)	tests in touched files
6	StreamingConfig.storage_backend with #[serde(default)]	asap-common/dependencies/rs/asap_types/src/streaming_config.rs

File domain

Touched only the files listed under "Touch ONLY" in the brief, plus the test-utility struct-literal sites (tests/test_utilities/{config_builders,engine_factories}.rs, tests/{capability_matching_tests,sql_pattern_matching_tests}.rs, tests/inference_yaml_pattern_coverage.rs) that needed the new struct field. No precompute_engine/, stores/, or precompute_operators/ modified. No submodule pointer drift.

Routing matrix

compatible_storage_backends:

metric_storage_config	accuracy	result
GorillaS3Archive	any	[GorillaS3Archive] (exact subsumes approximate)
SketchWarmTier (default)	any	[SketchWarmTier, ColdJsonlFallback]
DoubleWrite	Exact	[GorillaS3Archive, SketchWarmTier, ColdJsonlFallback]
DoubleWrite	Approximate	[SketchWarmTier, GorillaS3Archive, ColdJsonlFallback]
ColdJsonlFallback	any	[ColdJsonlFallback]

EngineRouter::execute walks this list in order, dispatches to first registered engine, falls through on EngineError::Backend or EngineError::CapabilityMiss. Returns EngineRouterError::NoEngineRegistered if nothing in the list has an engine; EngineRouterError::AllFailed if every engine errored.

Test plan

• cargo build --release -p asap_types clean
• cargo build --release -p query_engine_rust clean
• cargo test -p asap_types capability_matching — 36 pass (was 30; +6 routing)
• cargo test -p asap_types streaming_config — 2 pass (new)
• cargo test --release -p query_engine_rust router — 6 router tests pass
• cargo test --release -p query_engine_rust gorilla_engine — 20 pass (no regression)
• cargo test --release -p query_engine_rust capability_matching_tests — 6 pass (no regression)
• cargo clippy --release -p asap_types --all-targets -- -D warnings clean
• Pre-existing query_engine_rust clippy errors and 35 lib-test failures verified unchanged via git-stash baseline diff (failure set identical; +6 new passing tests for the router)

Open questions / follow-ups

• The HTTP driver (asap-query-engine/src/drivers/http/...) and main.rs still construct SimpleEngine directly and call handle_query — they don't yet consume the new EngineRouter. Wiring the router into the HTTP-driver dispatch (so production traffic routes through compatible_storage_backends) is a Phase-6 / follow-up PR, intentionally out of scope here per "Open question" in the brief.
• compatible_storage_backends currently ignores Statistic (no statistic is special-cased). Once Phase-6 adds statistic-specific routing rules (e.g. archive-only for Topk to avoid sketch error), the test matrix already enumerates every (stat, accuracy, backend) triple and will catch divergence.
• Design doc §8 currently sketches a 3-variant StorageBackend; this PR ships 4 variants (adds explicit DoubleWrite) — small doc-tightening to land alongside the deploy YAML overlay (§8.4).