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>

Infrastructure/Pcan/PcanAdapter.cs

@@ -64,8 +64,10 @@ namespace HC_APTBS.Infrastructure.Pcan
         private HashSet<uint> _pumpMessageIds = new();
         private DateTime _lastBenchFrameUtc = DateTime.MinValue;
         private DateTime _lastPumpFrameUtc = DateTime.MinValue;
-        private bool _benchAlive;
-        private bool _pumpAlive;
+        // volatile so IsPumpAlive/IsBenchAlive getters on other threads see transitions
+        // without relying on the memory-model guarantees of the event handler path.
+        private volatile bool _benchAlive;
+        private volatile bool _pumpAlive;
 
         // ── ICanService ──────────────────────────────────────────────────────────
 
@@ -84,6 +86,9 @@ namespace HC_APTBS.Infrastructure.Pcan
         /// <inheritdoc/>
         public bool IsConnected => !_stopRead;
 
+        /// <inheritdoc/>
+        public bool IsPumpAlive => _pumpAlive;
+
         /// <inheritdoc/>
         public TPCANHandle SelectedChannel
         {
@@ -246,11 +251,11 @@ namespace HC_APTBS.Infrastructure.Pcan
         {
             lock (_mapLock)
             {
+                // Replace-on-conflict: callers may re-register on pump switch; the
+                // new pump's parameter objects must take precedence over any stale
+                // objects from the previous pump that shared CAN IDs.
                 foreach (var kv in parameters)
-                {
-                    if (!_parameterMap.ContainsKey(kv.Key))
-                        _parameterMap.Add(kv.Key, kv.Value);
-                }
+                    _parameterMap[kv.Key] = kv.Value;
                 ResolveBenchRpmParam();
             }
         }
@@ -563,6 +568,11 @@ namespace HC_APTBS.Infrastructure.Pcan
                 // result = prev + alpha * (new - prev)
                 param.Value = PassFilterUpdate(previousValue, param.Value, param.Alpha);
                 param.NeedsUpdate = true;
+
+                // Notify observers (e.g. UnlockService) that the decoded value changed.
+                // The filter rounds to 4 decimals so this does not fire on float noise.
+                if (param.Value != previousValue)
+                    param.RaiseValueChanged();
             }
         }