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/sched/fake_engine_port.hpp

@@ -24,6 +24,7 @@ public:
     std::vector<vdm::TaskId> paused;
     std::vector<vdm::TaskId> resumed;
     std::vector<std::pair<vdm::TaskId, bool>> cancelled;
+    std::vector<vdm::TaskId> released;
     std::vector<std::vector<vdm::TaskId>> orders;
     std::vector<std::uint32_t> max_active_segments;
     std::vector<std::pair<std::string, std::uint32_t>> host_caps;
@@ -40,6 +41,7 @@ public:
     void provide_auth(vdm::TaskId, const std::string&, const std::string&, bool) override {}
     void decide(vdm::TaskId, vdm::task::Decision) override {}
     void refresh_url(vdm::TaskId, const std::string&) override {}
+    void release(vdm::TaskId id) override { released.push_back(id); }
     void set_task_order(const std::vector<vdm::TaskId>& order) override { orders.push_back(order); }
     void set_max_active_segments(std::uint32_t n) override { max_active_segments.push_back(n); }
     void set_host_segment_cap(const std::string& h, std::uint32_t c) override {