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/event_loop.hpp

@@ -12,7 +12,9 @@
 #include <atomic>
 #include <cstdint>
 #include <functional>
+#include <mutex>
 #include <unordered_map>
+#include <vector>
 
 namespace velox::daemon::rpc {
 
@@ -52,6 +54,10 @@ public:
     // callback. Async-signal-safe.
     void wake() noexcept;
 
+    // Run `fn` on the loop thread at the next iteration. Thread-safe; the intended way to
+    // marshal an engine-thread callback back onto the RPC loop.
+    void post(std::function<void()> fn);
+
 private:
     struct Entry {
         unsigned interest;
@@ -59,11 +65,15 @@ private:
     };
 
     void drain_wakeup() noexcept;
+    void drain_posts();
 
     int wake_fd_;                      // eventfd, always registered
     bool running_ = false;
     std::atomic<bool> stop_requested_ = false;   // set from stop(), read by run()
     std::unordered_map<int, Entry> fds_;
+
+    std::mutex post_mu_;
+    std::vector<std::function<void()>> posts_;
 };
 
 }  // namespace velox::daemon::rpc