Problem: parse_buffer classified /id/-prefixed task lines as headers
because '/' matches the '^%S' header pattern. Store timestamp test
was flaky when add and update ran within the same second.
Solution: check for task line patterns (id prefix or 2-space indent)
before falling through to the header branch. Backdate the initial
modified timestamp in the store update test.
Problem: need to reconcile buffer edits against the JSON store
on :w, handling creates, deletes, updates, reorders, and
duplicate IDs from yank/paste.
Solution: add diff module that parses buffer lines, matches
against stored tasks by ID, creates new tasks for unknown or
duplicate IDs, marks removed tasks as deleted, and updates
changed fields.
