FoR scheme bit-packs its child outside the allowed schemes #10751

What happened?

FoRScheme::compress bit-packs its encoded child by calling BITPACKING_V1.compress directly (vortex-btrblocks/src/schemes/integer/for_.rs:220). The child skips the compressor's scheme selection, and FoRScheme::produced_encodings declares only fastlanes.for (and fastlanes.for.v2), not the bit-packing IDs that the child uses. The compressor keeps a scheme only when every ID that it declares is allowed (vortex-btrblocks/src/builder.rs, allowed_schemes), so this check misses the bit-packed child.

From reading the code, this has two effects:

• If fastlanes.bitpacked is not allowed, or the bit-packing scheme is excluded with exclude_schemes, the FoR scheme still writes a bit-packed child. The writer then rejects the forbidden ID, and the write fails, although the compressor could have picked another scheme. With experimental patches enabled, the same applies to vortex.patched.
• When fastlanes.bitpacked.v2 is allowed, the FoR child still uses the v1 bit-packing variant.

Expected: FoR declares the IDs that its child can write, or compresses the child through the selected schemes.

Environment

develop at 3974ee069c. Found by reading the code, not reproduced by a run.

Additional context

Found while updating the compression section of the versioning docs (#10747).