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.cpp

@@ -45,6 +45,23 @@ void EventLoop::stop() noexcept {
     wake();
 }
 
+void EventLoop::post(std::function<void()> fn) {
+    {
+        std::lock_guard<std::mutex> lk(post_mu_);
+        posts_.push_back(std::move(fn));
+    }
+    wake();
+}
+
+void EventLoop::drain_posts() {
+    std::vector<std::function<void()>> batch;
+    {
+        std::lock_guard<std::mutex> lk(post_mu_);
+        batch.swap(posts_);
+    }
+    for (auto& fn : batch) fn();
+}
+
 void EventLoop::drain_wakeup() noexcept {
     std::uint64_t sink = 0;
     while (::read(wake_fd_, &sink, sizeof(sink)) > 0) {
@@ -87,6 +104,8 @@ void EventLoop::run() {
             if (p.revents != 0) fired.push_back(p.fd);
         }
 
+        drain_posts();
+
         for (const int fd : fired) {
             const auto it = fds_.find(fd);
             if (it == fds_.end()) continue;  // removed by an earlier callback this pass