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.