fix: gate Ford VP44 unlock on CAN liveness to prevent false-unlocked reads

Before this fix, StartUnlockIfRequired was called immediately after
registering the pump's CAN parameters, before any frames had been
decoded. The TestUnlock parameter's zero-initialised Value was
interpreted as "unlocked" for Type 1 pumps, causing Phase 1 to be
skipped and UnlockCompleted(true) to fire falsely.

Changes:
- ICanService: add IsPumpAlive property (volatile-backed in PcanAdapter)
- PcanAdapter: implement IsPumpAlive; mark _pumpAlive/_benchAlive volatile
  for safe cross-thread reads
- MainViewModel: replace direct StartUnlockIfRequired call with a
  fire-and-forget WaitForPumpCanThenUnlockAsync that waits for
  PumpLivenessChanged(true) + 250 ms grace, then invokes unlock on the
  UI thread; cancellation on pump change or CAN disconnect via
  _pumpLivenessCts
- UnlockService.UnlockAsync: skip Phase 2 state-machine when observer
  seed already reports unlocked (senders still run to prevent re-lock)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Services/Impl/UnlockService.cs

@@ -87,8 +87,19 @@ namespace HC_APTBS.Services.Impl
             ct.ThrowIfCancellationRequested();
 
             // ── Phase 2: TestUnlock state machine ────────────────────────────────
-            StatusChanged?.Invoke("Testing unlock...");
-            await RunTestUnlockSequenceAsync(pump.UnlockType, ct).ConfigureAwait(false);
+            // Skip when the observer already latched an unlocked state — the
+            // four 0x700 commands are a no-op in that case and just keep the
+            // dispatcher/CAN bus busy for ~4 s. Senders remain running so the
+            // Ford ECU doesn't re-lock.
+            if (_isPumpUnlocked)
+            {
+                _log.Info(LogId, "Pump already unlocked — skipping Phase 2 state machine");
+            }
+            else
+            {
+                StatusChanged?.Invoke("Testing unlock...");
+                await RunTestUnlockSequenceAsync(pump.UnlockType, ct).ConfigureAwait(false);
+            }
 
             // ── Verify unlock status via CAN TestUnlock parameter ────────────────
             bool success = VerifyUnlock(pump);