Add V7 raw forward index format for codec pipelines

[xiangfu0]

Stacked on #19306 (xiangfu0/codex/codec-stack/04-t64-gorilla). Part of the split of #18229.

Summary

Adds the V7 on-disk raw forward index format — the first format that can persist a codec
pipeline — plus the factory routing that decides, per codecSpec, whether a column can keep
using the legacy chunk formats or needs V7:

• FixedByteChunkForwardIndexWriterV7 (new, io.writer.impl): chunked SV fixed-byte writer
  for INT/LONG. File header carries version=7, an explicit FORMAT_MAGIC (0xC0DEC0DE)
  discriminator, chunk geometry, and the canonical codec spec (UTF-8) followed by a long[]
  chunk-offset table; each chunk stores encodedSize/decodedSize and the pipeline-encoded
  payload. The writer validates the pipeline's composed encoded-size bound and cumulative work
  bound up front so it never emits a file it could not read back.
• FixedByteChunkSVForwardIndexReaderV7 (new, readers.forward): self-describing reader
  that re-creates the CodecPipelineExecutor from the header string and fully validates the
  header (chunk geometry consistency, spec length, offset-table monotonicity/bounds, per-chunk
  sizes against the composed pipeline bound) before any allocation sized from file data.
• FixedByteValueWriter (new interface) + retrofit on FixedByteChunkForwardIndexWriter, so
  SingleValueFixedByteRawIndexCreator holds a single writer reference and gains a V7
  constructor taking a CodecPipelineExecutor.
• CodecSpecUtils (new, public, io.codec): classification helper —
  toLegacyChunkCompressionType(...) (compression-only specs that map 1:1 to a legacy
  ChunkCompressionType, e.g. LZ4, ZSTD(3)) and hasTransform(...). Alongside
  CodecPipelineExecutor this is the only public entry into the package-private codec runtime.
• ForwardIndexCreatorFactory: codecSpec routing — legacy-mappable compression-only specs
  go to the existing raw writers (byte-identical files to a legacy compressionCodec config);
  transforms/chains/non-default options go to the V7 writer (SV INT/LONG only, with
  defense-in-depth checks).
• ForwardIndexReaderFactory: V7 dispatch keyed on the second-int FORMAT_MAGIC check
  BEFORE the version >= 4 power-of-2 fallback (legacy fixed-byte writers accept arbitrary
  versions ≥ 4, so the version integer alone can never identify V7), plus a truncation check
  before reading the version int.
• SPI: ForwardIndexConfig.CODEC_PIPELINE_WRITER_VERSION = 7 and
  ForwardIndexReader.getCodecSpec() default method (V7 readers return the canonical header
  spec; every other reader returns null).
• ForwardIndexHandler guard (~6 lines): in shouldChangeRawCompressionType, if the
  existing reader reports getCodecSpec() != null, return false before the
  existingCompressionType != null precondition. V7 segments are never rewritten yet (the next
  PR owns reload/rewrite). The computeOperations gate already rejects codecSpec configs;
  this guard protects against V7 segments on disk (e.g. tooling-created) paired with a
  non-codec config transition.

Canonicalization (frozen into on-disk headers)

The V7 writer embeds executor.getCanonicalSpec() — never ForwardIndexConfig's stored string,
which is only a structural normalization (toDslString) and can differ (e.g. ZSTANDARD vs
ZSTD(3)). The reader re-creates the executor from the header string, so the header is always
in executor-canonical form (covered by testCanonicalSpecStoredInHeader).

Gate relocation (deliberate, explicit)

The codec stack's closed feature gate (codecSpec is not supported yet for column: %s) was
planted at every config/creator/reader/reload surface in #19284. This PR removes exactly two
factory-level gates because it makes those code paths real:

• ForwardIndexCreatorFactory.createIndexCreator — now routes instead of throwing.
• ForwardIndexReaderFactory.createIndexReader (both overrides) — now reads instead of throwing.

All config-surface gates remain: ForwardIndexType.validate,
ForwardIndexType.shouldCreateIndex, ForwardIndexType.createMutableIndex,
ForwardIndexHandler.computeOperations (rejectUnsupportedCodecSpecs), and the OPEN_STRUCT
sites. Net effect: no table config carrying codecSpec can validate, build a segment, create
a mutable index, or reload — so no V7 segment is creatable from config — but the format code
itself and its direct-construction tests work end to end.

Deliberately excluded (next PR: 06-reload-enable)

• Opening the feature gate (removing the remaining rejectUnsupportedCodecSpec sites) and
  table-config validation of codecSpec column types (validateCodecSpec).
• ForwardIndexHandler rewrite support for codec-pipeline segments (codecSpec change
  detection via canonical-spec comparison, legacy↔V7 conversions on reload).
• Mutable/realtime forward index support for codecSpec.

Why master stays safe with only this merged

• The gate keeps codecSpec unusable from any table config, so the new writer/reader can only
  be reached by direct construction (tests/tools). No production config can produce a V7 file.
• V7 reader dispatch triggers only on the 0xC0DEC0DE second-int magic, which no legacy writer
  emits; all legacy version ≥ 4 files keep their exact previous routing (locked by
  ForwardIndexReaderFactoryBackwardCompatTest, including golden base64 fixtures produced
  before this code existed).
• The handler guard only short-circuits columns whose on-disk index already reports a codec
  spec, i.e. V7 segments that could not exist via config today; legacy columns follow the exact
  previous logic.

Verification

./mvnw -q -T 1C install -DskipTests -Ppinot-fastdev -pl pinot-segment-spi,pinot-segment-local -am
./mvnw -q test -Ppinot-fastdev -pl pinot-segment-local \
  -Dtest=CodecPipelineForwardIndexTest,FixedByteChunkSVForwardIndexReaderV7CorruptionTest,\
ForwardIndexCreatorFactoryTest,ForwardIndexReaderFactoryTest,ForwardIndexReaderFactoryBackwardCompatTest,\
CodecSpecUtilsTest,CompressionCodecCorruptInputTest,ForwardIndexHandlerTest \
  -Dsurefire.failIfNoSpecifiedTests=false
# precommit
./mvnw spotless:apply license:format -pl pinot-segment-spi,pinot-segment-local
./mvnw checkstyle:check license:check -pl pinot-segment-spi,pinot-segment-local

Stack (split of #18229)

1. Fix forward index reader context lifecycle #19281 — Fix forward index reader context lifecycle (merged)
2. Fix delta forward index chunk caching #19282 — Fix delta forward index chunk caching (merged)
3. Add codec spec DSL and configuration plumbing #19284 — Codec spec DSL and configuration plumbing
4. Add bounded codec runtime and compression handlers #19285 — Bounded codec runtime and compression handlers
5. Add DELTA and DELTADELTA transform codecs #19305 — DELTA and DELTADELTA transform codecs
6. Add T64 and GORILLA packing transform codecs #19306 — T64 and GORILLA packing transform codecs
7. Add V7 raw forward index format for codec pipelines #19307 — V7 raw forward index format ← this PR
8. Support codecSpec on segment reload and enable codecSpec end to end #19308 — Reload/rewrite support, gate removal (enabling PR)
9. Add codec pipeline integration tests and design doc #19309 — Integration tests and design doc

Each PR is based on its predecessor's branch; GitHub retargets the next PR as each merges.