feat(transaction): experimental action-based transaction format skeleton

[wjones127]

Skeleton establishing the shape and safety gating of action-based
(UserOperation) transactions — the first slice of the Action-based
Transactions milestone (discussion #5960).

Stacked on #7670

This PR is stacked on #7670 (the general experimental-feature mechanism).
Its first commit belongs to #7670; review that PR first. Action-based
transactions are the first consumer of that mechanism rather than claiming a
dedicated feature-flag bit:

• A dataset using action-based transactions declares the "action-transactions"
  experimental feature, registered in known_experimental_features() only under
  the unstable-action-transactions Cargo feature.
• That sets FLAG_EXPERIMENTAL, so libraries that don't understand it fail
  closed. On graduation (PMC vote), the name moves onto its own dedicated bit.

Double gating

1. Cargo feature — everything is behind the non-default
   unstable-action-transactions feature, so it is not compiled into released
   builds at all.
2. Experimental feature flag — via FLAG_EXPERIMENTAL + the
   "action-transactions" name, as above.

What's here (the skeleton commit)

• protos/transaction_experimental.proto — UserOperation / UserAction /
  Action (with AddFragments only, as a proof of shape), in a separate proto
  file so canonical transaction.proto stays clean until promotion. Compiled
  only under the feature.
• lance_table::transaction — Rust UserOperation / UserAction / Action /
  AddFragments types with protobuf conversions, a roundtrip test, and the
  FEATURE_NAME constant.
• Registration of "action-transactions" in the experimental-feature registry,
  with admission tests for both default-build (rejected) and feature-on
  (recognized).

What's NOT here (follow-up vertical slices)

• Translation from Operation, apply, and conflict resolution.
• Wiring into the commit path / public API (nothing yet populates the
  experimental feature list).
• The remaining actions beyond AddFragments.

Part of #5960. Precedes the proto PMC vote (#6455), which will promote the
finalized messages into transaction.proto.

🤖 Generated with Claude Code

[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.