Add a strict expected-head dataset commit mode

Goal

Allow a caller to commit against one exact dataset head and fail if that head has changed, without performing an initial conflict rebase.

Motivation

Some publication workflows validate a complete snapshot-dependent basis before staging output. The current commit path may rebase a non-overwrite transaction before its first commit attempt, so a concurrent change can land after final validation and still be accepted as compatible.

The limitation was surfaced while evaluating https://github.com/lancedb/lancedb/pull/3997.

Contract

• The caller supplies an exact expected dataset version or equivalent head witness.
• The first commit attempt fails with a typed conflict if the current head differs.
• Strict mode performs no initial conflict rebase and no compatibility-based acceptance.
• Retries cannot weaken or replace the original expected-head witness.
• Default commit behavior remains unchanged for callers that do not request strict mode.
• Object-store and commit-handler implementations expose equivalent semantics.

Completion gates

• Local and object-store tests cover unchanged head, changed head, concurrent compatible changes, concurrent incompatible changes, and retries.
• The strict path cannot publish after any intervening dataset commit.
• Existing transaction conflict behavior remains unchanged outside strict mode.
• Rust and required binding surfaces document the exact failure contract.