daemon: persist engine progress/probe to the store — segments:0 no longer leaks

Engine numbers never reached the store: download.get after a correctly-finished
download reported sizeBytes: null, downloadedBytes: 0, speedBps: 0, resumable:
false, segments: 0, segmentDetail: [] — segments: 0 breaks the frozen contract
(TaskSummary.segments is minimum:1, required).

- Scheduler::tick() now probes (EnginePort::probe) before every start(), persisting
  sizeBytes/resumable/validators via Tasks::set_probe_result before a byte moves,
  then starts the engine with that ProbeResult as probe_hint.
- Scheduler::persist_progress() (new) writes downloadedBytes/speedBps/segments/
  segmentDetail from the engine's Progress. Called from progress_snapshot() (the
  250ms tick) *and* once more from on_engine_state right before release()/unmap on
  every terminal transition, so a task that finishes between two ticks — the common
  case for anything small or fast — still leaves real numbers instead of the
  pre-persistence defaults.
- TaskSummary.segments is sourced from segments.size() when the task has any
  (matching what actually lands in segmentDetail, per the schema's "exactly
  segments entries"), falling back to the engine's effective_segments (budget
  slots *held*, not necessarily physical range count) only pre-segmentation.
- Tasks::set_final_bytes tops up on_finished's byte count as a last-resort
  backstop.
- store/segments.{cpp,hpp}: read/write access to the segments table behind
  TaskDetail.segmentDetail. Wired into daemon/CMakeLists.txt.
- migrations/0002: speed_bps on tasks and segments; fixes segments.state's CHECK
  to include 'pending' (0001 omitted it, so a pre-connect snapshot could never be
  written).
- store_migrations_test's forward-only loop faked "released version N" by setting
  the user_version pragma alone, with no real schema underneath — never exercised
  until 0002 existed. Fixed to actually build the db through migrations 1..N first.

Verified against real veloxd + tools/testserver (not just unit tests):
download.list/download.get correct immediately after completion and after a
daemon restart, with saveTo.allowedRoots pointed at an isolated dir.

Observed but not fixed (CORE, not this lane, noted in deferrals.md): Progress.
speed_bps reads back 0 for the whole lifetime of a live throttled download in the
same E2E check, despite downloadedBytes visibly advancing. DAEMON passes it
through unmodified; filed rather than worked around.

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

daemon/src/sched/engine_port.hpp

@@ -7,10 +7,11 @@
 // wraps `vdm::Engine` + `vdm::segment::SegmentBudget`; FakeEnginePort records calls.
 //
 // Task ids here are `vdm::TaskId` — the engine assigns one from start() and the Scheduler
-// keeps the wire-UUID <-> TaskId map (ADR 0013). Admission is the Scheduler's: it calls
-// start() only for a task the governor admitted, and the engine begins probing at once
-// (it does not queue). The min-1 fairness rule in SegmentBudget then guarantees each
-// started task a slot; set_task_order pushes the priority.
+// keeps the wire-UUID <-> TaskId map (ADR 0013). Admission is the Scheduler's: for a task
+// the governor admits, it probes first (persisting sizeBytes/resumable/validator before a
+// single byte moves), then calls start() with that ProbeResult as probe_hint. The min-1
+// fairness rule in SegmentBudget then guarantees each started task a slot; set_task_order
+// pushes the priority.
 
 #include <cstdint>
 #include <functional>
@@ -19,7 +20,9 @@
 #include <vector>
 
 #include "vdm/ids.hpp"
+#include "vdm/net/probe.hpp"
 #include "vdm/task/download.hpp"
+#include "vdm/util/result.hpp"
 
 namespace velox::daemon::sched {
 
@@ -27,6 +30,13 @@ class EnginePort {
 public:
     virtual ~EnginePort() = default;
 
+    // Runs on the probe pool, outside the segment budget (ADR 0011 §5); `done` arrives on
+    // an engine thread, exactly once. The Scheduler probes before every start() so it
+    // always has a real ProbeResult (size, resumable, validator) to persist and to pass
+    // back as DownloadSpec.probe_hint — one code path instead of "sometimes has one".
+    virtual void probe(const vdm::net::ProbeRequest& req,
+                       std::function<void(vdm::Result<vdm::net::ProbeResult>)> done) = 0;
+
     virtual vdm::TaskId start(const vdm::task::DownloadSpec& spec,
                               vdm::task::DownloadCallbacks callbacks) = 0;