daemon: D4b — download.pause/resume/start/cancel and queue.start/stop drive the scheduler

download.pause/resume/start/cancel and queue.start/stop were stubs; now they call into
the scheduler and take effect immediately, not on the next 1s tick — pausing, resuming
or cancelling a live transfer can't wait, and per ADR 0013 §3 the governor never
touches a user-owned pause on its own.

New rpc::TaskActionPort interface (owned by rpc/, implemented by sched::Scheduler) is
what dispatcher.hpp depends on instead of sched/scheduler.hpp directly. Needed because
veloxd_sched already links veloxd_rpc (for EventHub); dispatcher.hpp pulling in
sched/scheduler.hpp directly would make it a real circular library dependency, breaking
anything that links veloxd_rpc alone (cli's tests, as it turned out — hit and fixed
during this change).

Scheduler::user_pause/user_resume/user_start/user_cancel + pause_queue follow tick()'s
existing to_pause pattern: call the engine (async, no synchronous effect) and transition
the store eagerly so download.get/list are correct the instant the RPC call returns.

Fixed a real bug surfaced while building this: transition() always overwrote
pause_reason to NULL when the engine's own delayed pause-ack callback (on_state to
paused, no error) arrived after whoever actually initiated the pause had already
written the real reason — now it preserves the stored reason when the callback supplies
none, instead of clobbering it. Covered by a regression check in sched_scheduler_test.

store/queues gets get() and set_state() (was list()-only) for queue.start/stop.

Verified against real veloxd + tools/testserver, not just unit tests: pausing a live
single-segment throttled transfer freezes downloadedBytes, resume continues it from
that point, cancel stops it; a bad taskId comes back in BulkTaskResult.failed with
-32010, not a crash; queue.stop(pauseRunning:true) pauses the queue's running task
immediately and queue.start resumes admission.

Known gap: download.start's contract "a task in 'queued' jumps its queue" (priority
bump) is not implemented — admission is still plain FIFO by created_at. Noted in
deferrals.md.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GRDjHGgpYmMoPE2UFbe7pP

daemon/src/store/queues.hpp

@@ -5,6 +5,8 @@
 // — membership changes through download.update / queue.reorder, per Queue's own schema
 // note that a queue.upsert payload's taskIds is ignored.
 
+#include <optional>
+#include <string_view>
 #include <vector>
 
 #include "store/sqlite.hpp"
@@ -18,6 +20,13 @@ public:
 
     DbResult<std::vector<velox::proto::Queue>> list();
 
+    // nullopt (not an error) if no queue has this id.
+    DbResult<std::optional<velox::proto::Queue>> get(std::string_view queue_id);
+
+    // 'running' or 'stopped' (Queue.schema.json / the state column's CHECK). false if the
+    // id doesn't exist.
+    DbResult<bool> set_state(std::string_view queue_id, std::string_view state);
+
 private:
     Db& db_;
 };