B2.3: migrate MGC to LatestWinsGate.fireAndWait

specificClass.js 319 → 311 lines. Removed inline _dispatchInFlight +
_delayedCall + finally block. handleInput is now a 1-line delegate
to DemandDispatcher.fireAndWait({source, demand, ...}).
turnOffAllMachines calls _demandDispatcher.cancelPending().
DemandDispatcher 39 → 53 lines. One integration test rewritten to
use the new sentinel-resolution semantics. 77/77 tests pass.

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

src/dispatch/demandDispatcher.js

@@ -26,10 +26,25 @@ class DemandDispatcher {
         this._gate.fire(demand);
     }
 
+    // Returns a promise that resolves when THIS demand's dispatch settles.
+    // If superseded by a later fireAndWait while parked, the promise
+    // resolves with the LatestWinsGate SUPERSEDED sentinel
+    // ({ superseded: true }) — callers can branch on it without try/catch.
+    fireAndWait(demand) {
+        return this._gate.fireAndWait(demand);
+    }
+
     drain() {
         return this._gate.drain();
     }
 
+    // Cancels any parked pending value so it cannot run. The currently
+    // in-flight dispatch (if any) still runs to completion. A parked
+    // fireAndWait promise resolves with the SUPERSEDED sentinel.
+    cancelPending() {
+        if (this._gate._pending) this._gate._supersedePending();
+    }
+
     get inFlight() {
         return this._gate.size > 0;
     }