feat: add ParquetMergePipeline supervisor and publisher feedback (Phase 3d+3e)

Phase 3 pipeline integration, combined supervisor and feedback PR:

- ParquetMergePipeline supervisor: spawns all merge actors (publisher →
  sequencer → uploader → executor → downloader → planner), health-checks
  with periodic supervision loop, respawn on failure with backoff,
  graceful shutdown via FinishPendingMergesAndShutdownPipeline that
  disconnects feedback and runs finalize policy. 3 tests.

- Publisher feedback: add parquet_merge_planner_mailbox_opt to Publisher
  (feature-gated behind cfg(feature = "metrics")). After successful
  ParquetSplitsUpdate publish of new ingested splits, sends ParquetNewSplits
  to the planner. Merge outputs (non-empty replaced_split_ids) are not
  fed back to avoid infinite loops.

- DisconnectMergePlanner extended to clear both Tantivy and Parquet planner
  mailboxes, supporting shutdown drain for both pipeline types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: g-talbot

quickwit/quickwit-indexing/src/actors/log_publisher_impl.rs

@@ -41,6 +41,10 @@ impl Handler<DisconnectMergePlanner> for Publisher {
     ) -> Result<(), ActorExitStatus> {
         info!("disconnecting merge planner mailbox");
         self.merge_planner_mailbox_opt = None;
+        #[cfg(feature = "metrics")]
+        {
+            self.parquet_merge_planner_mailbox_opt = None;
+        }
         Ok(())
     }
 }

quickwit/quickwit-indexing/src/actors/metrics_pipeline/mod.rs

@@ -27,6 +27,7 @@ mod parquet_doc_processor;
 mod parquet_indexer;
 mod parquet_merge_executor;
 pub(crate) mod parquet_merge_messages;
+mod parquet_merge_pipeline;
 mod parquet_merge_planner;
 mod parquet_merge_split_downloader;
 mod parquet_packager;
@@ -50,6 +51,7 @@ pub use parquet_doc_processor::{
 pub use parquet_indexer::{ParquetIndexer, ParquetIndexerCounters, ParquetSplitBatch};
 pub use parquet_merge_executor::ParquetMergeExecutor;
 pub use parquet_merge_messages::{ParquetMergeScratch, ParquetMergeTask, ParquetNewSplits};
+pub use parquet_merge_pipeline::{ParquetMergePipeline, ParquetMergePipelineParams};
 pub use parquet_merge_planner::ParquetMergePlanner;
 pub use parquet_merge_split_downloader::ParquetMergeSplitDownloader;
 pub use parquet_packager::{ParquetBatchForPackager, ParquetPackager, ParquetPackagerCounters};