Default disable_transaction_file once v1.0.0+ readers are widespread

Background

The transaction is already embedded inline in the manifest (transaction_section) on every commit, so the separate _transactions/ file is redundant. FLAG_DISABLE_TRANSACTION_FILE (bit 32) lets a writer skip it and save one write per commit. Exposing a public toggle is tracked in #7486.

Inline-transaction writing and reader-side support for the flag shipped in v1.0.0 (2025-12-12). A dataset written with the external file disabled cannot be read by releases before v1.0.0 (they don't understand bit 32 and will reject it).

Ask

Once v1.0.0+ readers are sufficiently widespread (the usual ~3-6 month window after release, i.e. roughly mid-2026), flip the default so disable_transaction_file is on by default and we stop writing the redundant external file.

Before flipping:

• Confirm read support for inline transaction_section has been stable across Rust, Python, and Java since v1.0.0.
• Make sure the toggle from #7486 still allows opting back in to writing the external file for users who need older-reader compatibility.

See the Feature Maturity doc for the rationale and the downgrade-floor framing.

Blocked by #7486.

Start with the Feature Maturity document and the implementation and reader-support changes described for v1.0.0, then review the toggle planned in #7486. Confirm stable inline transaction support across Rust, Python, and Java and verify that opting back into the external file remains possible before changing the default; done means the compatibility checks pass and older-reader users retain an escape hatch.