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/store_spec.lua

@@ -121,6 +121,27 @@ describe('store', function()
       local updated = store.get(t.id)
       assert.is_not_nil(updated['end'])
     end)
+
+    it('does not overwrite id or entry', function()
+      store.load()
+      local t = store.add({ description = 'Immutable fields' })
+      local original_id = t.id
+      local original_entry = t.entry
+      store.update(t.id, { id = 999, entry = 'x' })
+      local updated = store.get(original_id)
+      assert.are.equal(original_id, updated.id)
+      assert.are.equal(original_entry, updated.entry)
+    end)
+
+    it('does not overwrite end on second completion', function()
+      store.load()
+      local t = store.add({ description = 'Complete twice' })
+      store.update(t.id, { status = 'done', ['end'] = '2026-01-15T10:00:00Z' })
+      local first_end = store.get(t.id)['end']
+      store.update(t.id, { status = 'done' })
+      local task = store.get(t.id)
+      assert.are.equal(first_end, task['end'])
+    end)
   end)
 
   describe('delete', function()