Add codec pipeline integration tests and design doc

[xiangfu0]

Context

Stacked on #19308 and the final test/documentation slice of the split of #18229.

Integration coverage

CodecPipelineIntegrationTest builds and queries an offline table with representative pipelines covering all eight built-in codecs on INT/LONG raw columns:

• Legacy-compatible compression-only specs: LZ4, ZSTD(3), SNAPPY, GZIP.
• V7 compression-handler chains: DELTA,SNAPPY and DELTA,GZIP.
• Transform/packing pipelines including DELTA,LZ4, DELTA,ZSTD(3), DELTADELTA,LZ4, T64, GORILLA, T64,LZ4, and GORILLA,ZSTD(3).

Both SSE and MSE exercise sums, filters, batched point lookups across real chunk boundaries, and cross-codec consistency. The test also verifies dictionary coexistence and a raw STRING column with ZSTD(3).

The test opens every generated segment to prove the config is not ignored: V7 columns must use FixedByteChunkSVForwardIndexReaderV7 and expose the exact canonical spec; legacy-compatible numeric/string columns must retain the expected legacy reader and ChunkCompressionType; the dictionary column must remain dictionary encoded.

Design documentation

Adds docs/design/codec-pipeline-v7.md covering:

• The exact bounded grammar (4096-character cap and no leading-zero numeric arguments).
• Structural config normalization versus semantic executor/V7 canonicalization.
• Actual runtime placement in pinot-segment-local.
• V7 framing, exact byte/bit order, codec-library framing, corruption defenses, and golden fixtures.
• Offline and committed-realtime support versus the mutable consuming representation.
• Migration/rollback limits, mixed-version safety, resource bounds, and context-owned reusable decode scratch.

Safety and verification

This slice changes only integration tests and documentation. The final reactor run builds all 63 required modules and runs all 88 integration cases. Spotless, Checkstyle, license format, and license check cover every affected module in the stack.

Stack

#19284 → #19285 → #19305 → #19306 → #19307 → #19308 → #19309 (this PR)

Review and merge parent-first.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.53%. Comparing base (d0afba2) to head (4ed1bd6).

Additional details and impacted files

@@                              Coverage Diff                               @@
##             xiangfu0/codex/codec-stack/06-reload-enable   #19309   +/-   ##
==============================================================================
  Coverage                                          57.53%   57.53%           
  Complexity                                             7        7           
==============================================================================
  Files                                               2688     2688           
  Lines                                             164285   164285           
  Branches                                           26676    26676           
==============================================================================
  Hits                                               94523    94523           
+ Misses                                             61750    61749    -1     
- Partials                                            8012     8013    +1     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (?)
java-25	57.53% <ø> (ø)
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	57.53% <ø> (ø)
unittests	57.53% <ø> (ø)
unittests1	57.53% <ø> (ø)

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