feat: Phase 3d+3e — ParquetMergePipeline supervisor + publisher feedback

[g-talbot]

Summary

Stacked on #6358 (Phase 3c). Adds the supervisor and feedback loop that make the merge pipeline runnable.

• ParquetMergePipeline supervisor: spawns all merge actors (publisher → sequencer → uploader → executor → downloader → planner), periodic health-check supervision loop, respawn on failure with backoff, graceful shutdown via FinishPendingMergesAndShutdownPipeline that disconnects feedback and runs finalize policy for cold windows.
• Publisher feedback: adds parquet_merge_planner_mailbox_opt to Publisher (feature-gated cfg(feature = "metrics")). After successful ParquetSplitsUpdate publish of new ingested splits, sends ParquetNewSplits to the planner. Merge outputs are not fed back (guards against infinite loops).
• DisconnectMergePlanner extended to clear both Tantivy and Parquet planner mailboxes.

Note for reviewers: spawn concurrency semaphore

The Parquet merge pipeline has its own SPAWN_PIPELINE_SEMAPHORE (limit 10), separate from the Tantivy merge pipeline's semaphore. This means up to 10 Parquet pipelines and 10 Tantivy pipelines could be spawning simultaneously (20 total), each hitting the metastore. This may be desirable for isolation (one pipeline type can't starve the other's spawns), but reviewers should decide if a shared semaphore would be better for metastore protection. Note that actual merge execution concurrency is already shared via MergeSchedulerService's single semaphore.

Test plan

• 3 pipeline tests (spawn+supervise, shutdown drain, initial splits)
• 3 existing publisher tests pass (no regression)
• Compiles with and without metrics feature
• cargo clippy clean, license headers OK

🤖 Generated with Claude Code

[mattmkim]

LGTMing to unblock, but some comments