P4 wave 1: extract MGC concerns into focused modules

src/groupOps/        groupOperatingPoint + groupCurves (pure functions)
  src/totals/          totalsCalculator (dynamic + absolute + active)
  src/combinatorics/   pumpCombinations (validPumpCombinations + checkSpecialCases)
  src/optimizer/       bestCombination (CoG) + bepGravitation (BEP-G + marginal-cost)
  src/efficiency/      groupEfficiency (calc + distance helpers)
  src/dispatch/        demandDispatcher (LatestWinsGate-based; replaces
                       _dispatchInFlight + _delayedCall)
  src/commands/        canonical names from start (set.mode/scaling/demand,
                       child.register) + legacy aliases
  CONTRACT.md          inputs/outputs/events surface

53 basic tests pass (52 new + 1 pre-existing).
specificClass.js / nodeClass.js untouched — integration in P4 wave 2.

Findings flagged via agents (TODO append to OPEN_QUESTIONS.md):
  - calcGroupEfficiency.maxEfficiency is actually the mean (misleading name)
  - checkSpecialCases has a no-op `return false` inside forEach
  - MGC doesn't route cmd.startup/shutdown/estop — confirm if station broadcasts need it

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

src/dispatch/demandDispatcher.js

@@ -0,0 +1,38 @@
+'use strict';
+
+const { LatestWinsGate } = require('generalFunctions');
+
+// Thin wrapper around LatestWinsGate for the MGC demand path. Replaces
+// the original `_dispatchInFlight` + `_delayedCall` pair in
+// specificClass.handleInput: a new demand arriving while a dispatch is
+// in flight overwrites any pending one, so the latest value always wins
+// and intermediates are dropped silently.
+
+class DemandDispatcher {
+    constructor(ctx = {}, runFn) {
+        if (typeof runFn !== 'function') {
+            throw new TypeError('DemandDispatcher requires a runFn');
+        }
+        this.ctx = ctx;
+        this.logger = ctx.logger || null;
+        this._runFn = runFn;
+        this._gate = new LatestWinsGate(
+            async (demand) => this._runFn(demand, this.ctx),
+            { logger: this.logger },
+        );
+    }
+
+    fire(demand) {
+        this._gate.fire(demand);
+    }
+
+    drain() {
+        return this._gate.drain();
+    }
+
+    get inFlight() {
+        return this._gate.size > 0;
+    }
+}
+
+module.exports = DemandDispatcher;