daemon: wire the engine into veloxd — the vertical slice runs end to end

CORE stage 8 merged, so vdm::Engine is linkable. This closes D4a and
narrows D4b: `velox add <url>` now actually downloads.

- sched/engine_port_core.hpp — the real EnginePort: forwards to a live
  vdm::Engine, keeps the DownloadHandle per task for pause/resume/
  cancel/provide_auth/decide/refresh_url, drives set_task_order /
  set_max_active_segments / set_host_segment_cap via
  engine.segment_budget(). CORE confirmed the admission model: DAEMON
  decides when to start(); the engine's own download_task calls
  register_task/set_want internally — DAEMON never touches per-task
  budget calls. EnginePort gains release(TaskId) so the port drops a
  handle when the task goes terminal.
- rpc/event_loop — EventLoop::post(fn): thread-safe, runs fn on the
  loop thread next iteration. The marshaller for engine-thread
  callbacks.
- main.cpp — constructs vdm::Engine + EnginePortCore + Scheduler
  (post_to_loop = loop.post). At startup: reconcile_after_restart()
  (ADR 0013 §5), reload_config(), tick(). A 1 s timerfd on the loop
  re-runs tick() (schedule windows, missed nudges); download.add nudges
  via dispatcher.set_on_mutation.

End-to-end verified against tools/testserver: `velox add
http://127.0.0.1:.../file/512K` -> task queued -> scheduler admits ->
engine downloads 524288 bytes -> complete, file on disk. First
byte-path all the way through the project.

safepath-adversarial.md: re-verified per its own note — CORE landed
O_NOFOLLOW on the target open (core/src/io/sparse_file.cpp), so the
leaf-symlink TOCTOU is now closed; residual is down to one
intermediate-dir gap (documented post-M1 chase).

36 daemon/cli tests green; scheduler + uds_roundtrip TSan-clean.

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

daemon/src/rpc/dispatcher.hpp

@@ -12,6 +12,8 @@
 // "not implemented in this build" (-> -32603) until its handler and the scheduler land;
 // see daemon/docs/deferrals.md.
 
+#include <functional>
+
 #include "store/sqlite.hpp"
 #include "velox_proto.hpp"
 
@@ -21,6 +23,10 @@ class VeloxDispatcher final : public velox::proto::Dispatcher {
 public:
     explicit VeloxDispatcher(velox::daemon::store::Db& db) : db_(db) {}
 
+    // Called after a handler mutates task state (download.add for now). main.cpp wires it
+    // to nudge the scheduler; unset in tests.
+    void set_on_mutation(std::function<void()> fn) { on_mutation_ = std::move(fn); }
+
     velox::proto::HandlerResult<velox::proto::CaptureRules>
     on_capture_getRules(const velox::proto::CaptureGetRulesParams&) override;
     velox::proto::HandlerResult<velox::proto::CaptureOfferResult>
@@ -101,6 +107,7 @@ public:
 
 private:
     velox::daemon::store::Db& db_;
+    std::function<void()> on_mutation_;
 };
 
 }  // namespace velox::daemon::rpc