* test: add top-priority missing test coverage
Problem: several critical code paths had zero test coverage —
parse.resolve_date (relative date resolution), store.snapshot
(foundation of the undo stack), and the diff.apply invariant that
unchanged tasks do not get their modified timestamp bumped. The
diff.apply due/priority clearing paths were also untested.
Solution: add six targeted test blocks across parse_spec, store_spec,
and diff_spec: resolve_date happy/failure paths, parse.body with
relative due tokens, snapshot copy-semantics and deleted-task
exclusion, diff unchanged-modified invariant, due cleared on removal,
priority cleared on ! removal.
* test: add second batch of missing test coverage
Problem: six more gaps from the audit remained after the first batch —
archive persistence verification, diff modified-on-rename, parse_buffer
inline cat:/due: token parsing, and store.update immutability invariants.
Solution: add six it() blocks across archive_spec, diff_spec, and
store_spec: archive unload/reload persistence check, modified timestamp
updated on description change, inline cat: overrides header category,
inline due: token parsed from buffer line, id/entry fields immutable
under store.update, and end timestamp not overwritten on second
completion.
8e16744ebe