Add V7 raw forward index format for codec pipelines

[xiangfu0]

Context

Stacked on #19306 and part of the split of #18229.

Summary

This is the first slice that consumes codecSpec in production forward-index I/O.

• Adds the self-describing V7 fixed-byte SV INT/LONG writer and reader with an explicit 0xC0DEC0DE discriminator and canonical codec spec in the header.
• Routes a single legacy-compatible compression invocation through the established raw formats; transforms, chains, and non-default options route to V7.
• Recreates the executor from the persisted canonical spec on read.
• Keeps all legacy fixed-byte versions, including historical version 7, on their existing reader path unless the new discriminator is present.

Hardening

• The writer truncates reused output files, validates actual and composed bounds, caps cumulative pipeline work, and rejects executor/type/entry-size or putInt/putLong mismatches before corrupting state.
• The reader enforces the 4096-byte spec cap, exact contiguous chunk extents from the offset table through EOF, chunk-count/document-count consistency, per-frame bounds, and exact decoded sizes.
• The production factory passes segment metadata totalDocs; the reader rejects a mismatched header during construction rather than at first query.
• Each reader context owns reusable ping-pong decode scratch and closes it with the context.

V7 can now be created and loaded. Until #19308, reload does not reconcile codecSpec changes.

Verification

• V7 round trips for all runtime codecs and representative chains.
• Corrupt header, gap/trailing-byte, chunk-count, bounds, type mismatch, and wrong-writer-method tests.
• Production factory metadata-mismatch coverage.
• A hand-authored transform-only V7 golden fixture that pins header fields, offsets, decoded values, and exact writer reproduction.
• Legacy reader compatibility fixtures for prior fixed-byte writer versions.
• Spotless, Checkstyle, license format, and license check on the affected modules.

Stack

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

Review and merge parent-first.