[Subscription] Index consensus ACK progress by writer

[Caideyipi]

Description

ConsensusSubscriptionCommitState.stageCommittedAndAdvance previously found the next committed and outstanding entries by scanning the full backlog maps. With N outstanding entries from one writer and reverse-order ACKs, every ACK repeatedly scanned O(N) entries, producing a real O(N^2) ACK path.

I reproduced the behavior by calling the production ConsensusSubscriptionCommitState with a single writer:

backlog	baseline reverse-ACK time	indexed time
2,000	39.749 ms	3.116 ms
4,000	152.420 ms	5.222 ms
8,000	1,116.485 ms	5.737 ms

Each value is the best of three warmed runs. The baseline grows super-linearly as the backlog doubles; the indexed path does not. The new per-writer TreeSet indexes preserve the existing slot maps for exact ACK removal and make successor lookup logarithmic.

Remote-change check:

• Limit subscription in-flight event accumulation #18055 is still open and adds retained/in-flight event backpressure; it does not change ACK successor scans.
• Merged [Subscription] Reduce consensus WAL replay and ACK contention #18402/[Subscription] Reduce consensus WAL replay and ACK contention (#18402) #18527 reduce WAL replay and ACK lock contention, and merged [Subscription] Reduce consensus commit warning log noise #18450 reduces warning-log amplification; none indexes the commit-state scans.

Testing

• mvn -Ddevelocity.off=true spotless:apply -pl iotdb-core/datanode
• mvn -Ddevelocity.off=true -pl iotdb-core/datanode checkstyle:check -DskipTests
• ConsensusSubscriptionCommitStateTest: 19/19 passed, including a 2,048-entry reverse-ACK regression case.
• git diff --check

The normal full DataNode compile is currently blocked by existing workspace SNAPSHOT/source inconsistencies (SubscriptionQueueAdmission and related classes) and a Windows compiler native-memory failure; the changed manager and targeted test were compiled directly against the existing DataNode build artifacts.

Changed files

• ConsensusSubscriptionCommitManager
• ConsensusSubscriptionCommitStateTest