test: add missing coverage (#19)

* 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.

spec/archive_spec.lua

@@ -128,4 +128,13 @@ describe('archive', function()
     assert.is_true(descs['Keep pending'])
     assert.is_true(descs['Keep recent done'])
   end)
+
+  it('persists archived tasks to disk after unload/reload', function()
+    local t = store.add({ description = 'Archived task' })
+    store.update(t.id, { status = 'done', ['end'] = '2020-01-01T00:00:00Z' })
+    pending.archive()
+    store.unload()
+    store.load()
+    assert.are.equal(0, #store.active_tasks())
+  end)
 end)