extract collection directory into a second binary sharing the same build tree. subscribes to relay firehose, maintains its own RocksDB index, and serves listReposByCollection on port 2510. - build.zig: add collectiondir executable target - Dockerfile.runtime: copy collectiondir binary - collection_index: add default CF handle + getMeta/putMeta/deleteMeta - collectiondir.zig: HTTP server, health checks, admin endpoints - collectiondir_subscriber.zig: firehose consumer with cursor in RocksDB - collectiondir_backfill.zig: RocksDB-backed progress, no postgres all state in RocksDB — no shared postgres dependency. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>