Multi-file write transactions are rejected; there is no super-journal 2PC

Replaces #944, whose body described a silent split-commit window across ATTACHed databases. That window is gone.

A transaction that writes more than one file-backed database is **rejected** with `atomic commit across multiple file-backed databases is not supported` and rolled back in full. This includes TEMP triggers that write `main` while changing an attached file. Single-file writes and transactions involving a `:memory:` attachment still work.

Documented in README SQLite Compatibility and `test/sqlite_compatibility_contract.tsv` (`transaction.multifile_write`). Testfixture assertions that expect SQLite's super-journal two-phase commit cite this as `unsupported`.

Two-phase commit across ATTACHed files is not planned. Do not re-open a silent-split tracker; the write never starts.

Start with the README SQLite Compatibility section and test/sqlite_compatibility_contract.tsv, especially transaction.multifile_write. Verify that the unsupported behavior and testfixture assertions consistently describe rejection and full rollback; the payload does not identify a missing documentation change.

Written by the indexing model from the issue text.