Gate compat fixture writes with editions - #10077

Summary

Compat fixtures are written with disable_editions(), so a fixture can record an encoding with no read-compatibility guarantee. Once published, the additive-only manifest rule makes that encoding permanent. This PR writes fixtures under the default session's enabled editions (core2026.08.3), so a fixture can only emit encodings an edition already guarantees.

Changes

• Gated writes. adapter::write_compressed and adapter::write_compressed_to_bytes_with_session no longer call disable_editions(). check regenerates its comparison files through the same path, so it is gated too.
• Dataset compressor. TPC-H and ClickBench fixtures build their compressor and write strategy from VortexSession::default(), the session the file is written with, instead of from array_session() with disable_editions(). Since Filter compressor schemes by the session's allowed serialized IDs on build #10067, BtrBlocksCompressorBuilder::from_session keeps only schemes whose encodings the session registers and its editions allow. The compressor therefore can't choose an encoding the gated write would reject, such as vortex.patched under VORTEX_EXPERIMENTAL_PATCHES. The regular and compact variants now share one code path.
• decimal_byte_parts_v2.vortex disabled, like delta and patched. It writes vortex.decimal_byte_parts.v2, which no edition includes yet. It isn't in any published manifest (checked through 0.86.1), so removing it doesn't break the additive-only rule. It comes back once DBP v2 joins an edition.
• DESIGN.md notes that new fixtures must use encodings from an enabled edition.

Every fixture in the 0.86.1 manifest uses core encodings. That comes from reading the code, not from a run, so the published fixture set should be unchanged.

Checks

• cargo clippy -p vortex-compat --all-targets --all-features -- -D warnings
• cargo +nightly-2026-09-10 fmt -p vortex-compat --check
• cargo nextest run -p vortex-compat
• cargo run -p vortex-compat --release -- generate --output /tmp/compat --exclude clickbench: confirms every remaining fixture writes under core