refactor(table): move transaction.rs into lance-table

[wjones127]

Stacked on #8053. Review that one first; this PR targets its branch.

Building a manifest from a transaction reads and writes only table metadata. Now that #8053 has moved the helpers it depended on, transaction.rs has no dependency on Dataset, Session, DataFusion, or async I/O — restore_old_manifest is the only thing that touches storage at all, and it uses only ObjectStore and CommitHandler. So the file moves to lance-table.

The file itself is unchanged apart from import paths. lance::dataset::transaction becomes a re-export, so nothing changes at any call site — not in lance, not in the Python bindings, not in the Java bindings.

Git reports the move as a 97% rename, so the review surface is the import block rather than 6500 added and 6500 deleted lines. Keeping it that way is why the file arrives here whole and gets split into a module tree in a separate follow-up, and why the shim is an inline pub mod transaction in dataset.rs rather than a file at the old path — a file there would have left the addition with nothing to pair against, and the rename would not have been detected.

Beyond import paths, the moved file changes in three ways, all visible in the rename diff:

• build_manifest, restore_old_manifest, modifies_same_metadata and upsert_key_conflict widen from pub(crate) to pub, because their callers in io/commit.rs and io/commit/conflict_resolver.rs are now in another crate. lance's own public API is unchanged.
• The test module gains a default_build_config() helper, since ManifestWriteConfig stays in lance and its Default is what the 22 build_manifest test call sites used.
• One comment that named ManifestWriteConfig::default() now describes the default config without naming a type from a higher crate.

The one test that needed Dataset moves to dataset_transactions.rs in the first commit, so the second commit is the rename alone. Test counts confirm nothing was dropped: 58 tests before, 57 in lance-table after, plus that one.

Not included

The split into a module tree — operation.rs, conflicts.rs, manifest_build.rs, proto.rs and friends — is the next PR in the stack. This PR leaves a single 6488-line file in lance-table.

Collapsing the new lance_table::transaction::Transaction with the existing lance_table::format::Transaction (a thin pb::Transaction wrapper whose doc comment says it exists so that "lance-table does not depend on higher layers" — the exact inversion this removes) would change the public CommitHandler trait signature, so it is left for later.

[geruh]

The rename lgtm!