daemon: store/ query layer — tasks + settings (prep for the download.add vertical slice)

The read/write surface the dispatcher handlers need, so download.add
persists and download.list / download.get project real rows when the
engine lands.

- store/tasks — TaskRow (1:1 with the schema), insert / get / remove /
  set_state / count, and list(filter, sort, offset, limit) that does
  all the WHERE / ORDER BY / LIMIT in SQL (M1 DoD: a 1000-row list
  never materialised client-side). Filter covers states / category /
  queue / case-insensitive filename+url substring / date range; sort
  is a whitelisted column + direction with NULLs last, default
  newest-first; the enum spellings in a state IN (...) come from
  proto::to_string, never from user text. to_summary() projects a row
  onto proto::TaskSummary including the flattened error block when the
  task failed / retry_wait / auto-paused.
- store/settings — key -> JSON-text with a built-in default table
  mirroring Settings.schema.json / ADR 0012; get_raw / set_raw /
  overrides plus typed get_int / get_string / get_string_array for the
  governor config and saveTo.allowedRoots. Full settings.get/set wire
  projection lands with those handlers.
- veloxd_store now links velox::proto + nlohmann_json for the
  projection.

Test veloxd.store_tasks: insert/get round trip, PK duplicate rejected,
the error-block projection, list total+paging+sort+every filter,
set_state pause_reason clear-on-unpause, remove, and settings default
vs override. ASan+UBSan and TSan clean; 34 daemon/cli tests green.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Upd9WhG9oppieig5nRDLig

daemon/tests/CMakeLists.txt

@@ -19,3 +19,4 @@ veloxd_test(ws_server         LIBS veloxd_rpc)
 veloxd_test(sched_window     LIBS veloxd_sched)
 veloxd_test(sched_governor   LIBS veloxd_sched)
 veloxd_test(safepath         LIBS veloxd_fs)
+veloxd_test(store_tasks      LIBS veloxd_store)