feat: add mixed data file version capability

[Xuanwo]

Summary

• reserve bit 256 for mixed V2 data file versions without adding it to the supported mask
• keep this layer refusing mixed manifests until the per-file storage contract lands
• add shared commit gates and sticky-capability plumbing needed for safe activation

Stack

1. feat: add mixed data file version capability #8580 - capability reservation and commit gates
2. feat: validate mixed V2 data file versions #8581 - capability activation, storage validation, and exact read dispatch
3. feat: support per-operation V2 write targets #8582 - per-operation write targets
4. feat: propagate exact V2 versions across dataset operations #8583 - dataset operation propagation
5. feat: target exact V2 versions in compaction #8584 - compaction targets and binary copy policy
6. feat: expose exact V2 versions in bindings #8585 - bindings, documentation, and rollout contract

Testing

• cargo test -p lance-table feature_flags
• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

• Start a vote following the Lance community voting process.
  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
• Once the vote passes, link the completed vote in this PR. It should not be
  merged until the vote is linked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[lance-gatekeeper]

✅ Gate recommendation: approve.

The restore target now passes both reader and writer compatibility gates before feature recomputation, and the regression that previously exposed flag stripping now passes. Bit 256 remains reservation-only, while the existing uniform-version storage contract is unchanged.