docs: update runbook + cold-engine design to 2-tier framing

[zzylol]

Summary

Step-1 of the JSONL deprecation refactor (backend PR ProjectASAP/ASAPQuery-backend#95 + collector PR #312) deleted the legacy JSONL cold-fallback leg (LocalFsColdStore, parse_jsonl, the gateway-side raw-tee, the ColdJsonlFallback StorageBackend variant). This follow-up propagates the change into the runbook + cold-engine design doc.

Changes

• docs/mvp-demo-runbook.md:

  • rewrite the §"Out of scope for the demo" / §"Open gaps" references to point at the surviving Gorilla archive engine instead of the deleted JSONL ground-truth path
  • drop the LocalFsColdStore row from §Component status; rename the GorillaS3ColdStore row to GorillaS3Store (Step-1 rename); point at s3_cost.rs instead of the renamed s3_cost_tracker
  • move the "Delete LocalFsColdStore + JSONL gateway raw-tee + cost_model cold-tier scan-bytes" item from §Planned to DONE
  • flag Step-2 (Prometheus-TSDB block format + Thanos store-gateway) as the path to full PromQL parity on the archive tier

• docs/design-gorilla-s3-cold-engine.md:

  • update §3 e2e architecture: 2-tier framing, drop the "raw-tee exporter (cold JSONL)" leg from the block diagram, drop the "cold-JSONL fallback (forwarding adapters / LocalFsColdStore) — UNCHANGED" branch, capability miss now surfaces as 404 not a JSONL scan
  • swap GorillaS3ColdStore → GorillaS3Store in the diagram
  • add the routing/ module reference (the dispatcher moved out of engines/router.rs in Step-1)

Test plan

• markdown renders cleanly
• no remaining JSONL references in the diagram

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