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.