[Performance] Restore aligned memtable flush fast path

[Caideyipi]

Description

The aligned memtable flush fix in f87c9c3e80c introduced active-column mapping into the common path even when no measurement was deleted. Wide aligned batches then pay for Integer column mappings and additional per-point branches during encoding.

This change keeps the deleted-measurement encoding path unchanged and dispatches to the pre-f87c9c3 tight loop only when the active schema is the original schema. Flushing memtables are immutable, so the identity check is stable throughout encoding.

Two focused unit tests verify that:

• flushes without deleted measurements skip column mapping;
• flushes with deleted measurements still use the existing mapped path.

Tests

• mvn spotless:apply -pl iotdb-core/datanode
• mvn test -pl iotdb-core/datanode -Dtest=MemTableFlushTaskTest
  • Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
• The targeted IoTDBPipeReceiverAutoCreateDisabledIT method was attempted locally, but the test cluster failed in setUp because ConfigNode did not become reachable. The test body was not entered; the failure is unrelated to this change.

[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.33%. Comparing base (f493700) to head (9fee974).

Files with missing lines	Patch %	Lines
...e/dataregion/memtable/AlignedWritableMemChunk.java	77.27%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18406      +/-   ##
============================================
- Coverage     43.50%   43.33%   -0.17%     
  Complexity      374      374              
============================================
  Files          5395     5395              
  Lines        385520   385608      +88     
  Branches      50160    50185      +25     
============================================
- Hits         167715   167114     -601     
- Misses       217805   218494     +689     

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