Records append is not rollback-safe; read_records accepts any JSON object #206

Problem

1. run/description_file.py _append_records appends straight to the live morphobricks.records.jsonl (O_APPEND, then write, flush, fsync). If any of these raises after bytes were written, publish_description restores only morphobricks.toml. The records file then keeps a torn line, or a full record naming a SHA-256 of a description that was rolled back. The existing failure test (tests/test_description_file.py, test_a_failed_append_puts_the_previous_file_back) replaces _append_records with a function that raises before any byte is written, so it never saw this.
2. read_records accepts any JSON object, for example {}. Spec docs/experiment-v1.md section 4.6 (Spec main 964f097) fixes three record types (reconfirmation, image_count_changed, removed), each with a fixed key set, and says a line of one type carries no key of another.

Reproduced on main (1c54f4e)

• A binary write that fails after half of the bytes, or after all of them, or a failing fsync, leaves the records file changed while the TOML is put back.
• read_records returns ({},) for a file holding {}.

Fix

• Build the new records file (old bytes plus new lines) in full in a synced temporary file and replace the live file with one atomic replace; on any failure restore both the description and the records bytes.
• read_records refuses a line that is not one of the three types with exactly that type's keys.
• Define the reader rule for a record whose SHA does not match the published file: the hashes are history, never compared with the file now published (later publications change the file without a record).