Retained headless witness checks for existing .b2rec replay

[delk73]

Is this retained headless witness-check pattern useful for Box2D?

Branch:
https://github.com/delk73/box2d-replay/tree/replay/retained-witness-lab

Commit:
delk73/box2d-replay@7b68fa2

The idea:

• run known replay scenarios without the GUI
• emit structured witness rows through the public C API replay path
• compare generated witnesses against checked-in expected witnesses
• expose the check through a focused make replay-check target
• add malformed payload-boundary regression coverage around replay record dispatch

The branch includes a small replay change: each record's declared payload boundary is checked before dispatch, and known opcode handlers must consume exactly the declared payload. Unknown opcodes with valid payloads are still skipped.

Validation on the branch:

• make replay-check: PASS
• cmake --build build-nosimd --target test -j2: PASS
• ./build-nosimd/bin/test: PASS

A few scope notes:

• This branch is a discussion artifact for a retained headless witness-check pattern.
• It currently wires samples/headless_replay into top-level CMake for ease of review.
• If this direction is useful, that wiring could be adjusted.