docs: add missing why-comments across all merge pipeline files

Per CODE_STYLE.md: comments should convey intent, not implementation.
Added explanations for num_merge_ops lineage, known_split_ids rebuild
heuristic, output dir isolation, empty merge handling, scratch dir
lifetime, permit Drop safety, publisher setter ordering, and feedback
loop guard conditions.

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

Author: g-talbot

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

@@ -267,6 +267,7 @@ impl MergeSchedulerService {
             } = PeekMut::pop(next_merge);
             // The permit is owned by the task and released via Drop when
             // the executor finishes, triggering PermitReleased back here.
+            // Drop-based release ensures the semaphore is freed even on panic.
             let parquet_merge_task = ParquetMergeTask {
                 merge_operation,
                 merge_permit,