chore: delete the superseded GORILLA1 format (Rust store + legacy archive leg)

[zzylol]

Summary

Deletes the GORILLA1 read side. The custom GORILLA1 container format is fully superseded by Path A2: agents emit XOR-chunk fragments → backend gorilla-merger → TSDB blocks → S3 → thanos-query, answered by ThanosQueryEngine (env ASAP_THANOS_QUERY_URL). Path A2 is verified end-to-end, so the "Phase δ deletes this leg after Path A2 is verified" cleanup is now done.

What was removed

• data_plane/src/storage_engines/gorilla_object_store/ in full — GorillaS3Store + decode_block (the GORILLA1 block decoder via the asap_gorilla crate), the in-process GorillaQueryEngine + ExactExecutor (archive_query.rs), the postings sidecar, and the GORILLA1-only S3 cost wrapper (s3_cost.rs) plus its /metrics append and /internal/s3_cost.csv endpoint (those counters were only ever populated by GorillaS3Store).
• The legacy archive leg in main.rs: the Ok(None) fallback that built GorillaS3Store / GorillaQueryEngine is gone. When ASAP_THANOS_QUERY_URL is unset the binary now registers the existing NoDataArchiveEngine stub on the archive slot.
• The now-unused asap-gorilla path-dep + the rust-s3 / lru crates from data_plane/Cargo.toml (and their transitive entries in Cargo.lock).
• Stale doc-comment references to the deleted symbols across query_engines/{mod.rs,routing/mod.rs,thanos_query_engine/*}, storage_engines/mod.rs, and fallback/metrics.rs.

Kept intact (coupling note)

• StorageBackend::GorillaObjectStore (in asap_types) is kept — it is the archive-tier routing slot that the live ThanosQueryEngine registers under (data_source_id = thanos_query, see forward.rs). Only the answering engine changed; the routing key, its YAML serialized form (gorilla_object_store), and the backend_storage_routing tests are untouched.
• The sketch / thanos query paths and the gorilla-merger story.

Build & test status

• cargo build -p data_plane --all-targets → green (binary + all test targets; only pre-existing warnings).
• Lib unit tests pass: routing (77), thanos (15), http-server (53 passed / 1 pre-existing ignored).

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