First step of the one-file-per-record split. `decisions_lib.parse_text` now
dispatches on a leading `---`: the legacy "many ## records per file, metadata on
a backtick line" form and the new "one record per file, YAML frontmatter" form
both produce the same Record.
This is the load-bearing move for the migration. Because the parser understands
both, the body-diff guard can parse the OLD form at the merge-base and the NEW
form at head -- both keyed on `key` -- so the commit that relocates 166 records
validates itself with no one-time bypass and no escape hatch.
Details:
- Frontmatter uses the SAME field vocabulary as the backtick line
(`superseded-by`, `stale-after`), so on-disk names don't shift mid-migration.
- YAML `None` (a key with no value) is preserved as "" rather than collapsing to
None, keeping the empty-vs-absent distinction `stale-after` depends on (#603).
- `active_files()` also walks `docs/decisions/records/**`, so both layouts can
coexist while the migration lands.
- Malformed frontmatter (unterminated fence, non-dict, YAML error) yields no
record rather than raising; the validator then reports it as a missing record.
timothy
e063fd065f