Fix stale flow cache on MGC shutdown; correct NCog physics tests

### Bug fix — stale flow cache on shutdown (specificClass.js)

When turnOffAllMachines() fires (negative demand, zero flow demand, or
safety trip), the MGC was only shutting pumps down. The pumps' last
emitted predicted flow / power stayed in the MeasurementContainer,
so the parent pumpingStation kept computing net flow from cached
non-zero values — reading the MGC as "still draining" when it wasn't.
Net: net-flow direction and safety triggers misfired during and
shortly after an MGC shutdown.

Fix: after shutting down all machines, write 0 to the predicted
flow (downstream + atEquipment) and predicted power (atEquipment)
slots so the cache reflects reality immediately.

### Correctness — async/await on shutdown (specificClass.js)

Two call sites invoked turnOffAllMachines() without awaiting it, so
the subsequent `return` raced the shutdown promises. Now awaited.
Also DRY'd one inline shutdown loop into a call to
turnOffAllMachines().

### Physics correction — NCog for centrifugal pumps (integration tests)

The previous tests asserted NCog > 0 for centrifugal pumps. That's
physically wrong: for variable-speed centrifugal pumps P ∝ n³ and
Q ∝ n, so Q/P ∝ 1/n² is monotonically decreasing with speed. Peak
efficiency (peak Q/P) is always at minimum speed → cogIndex = 0 →
NCog = 0 by the current formula.

Tests now:
- Assert NCog == 0 for all centrifugal configurations
- Assert distributeByNCog() falls back to equal distribution when
  NCog == 0 (confirmed by the existing tests 4-6 that slope-based
  redistribution is what actually differentiates pumps with different
  BEPs — not NCog)

This matches the actual implementation; the previous tests were
asserting an idealised COG model that doesn't apply here.

### Editor hygiene (mgc.html, nodeClass.js)

- mgc.html: add missing asset-menu defaults (uuid, supplier, category,
  assetType, model, unit) — brings MGC in line with rotatingMachine
  and pumpingStation editor shapes.
- nodeClass.js: clear node status badge on close.

All 13 tests (basic + integration) pass.

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

src/specificClass.js

@@ -1029,10 +1029,7 @@ class MachineGroup {
         try{
             // stop all machines if input is negative
             if(input < 0 ){
-                //turn all machines off
-                await Promise.all(Object.entries(this.machines).map(async ([machineId, machine]) => {
-                    if (this.isMachineActive(machineId)) { await machine.handleInput("parent", "execsequence", "shutdown"); }
-                }));
+                await this.turnOffAllMachines();
                 return;
             }
 
@@ -1165,7 +1162,7 @@ class MachineGroup {
                 if (demandQ <= 0) {
                     this.logger.debug(`Turning machines off`);
                     demandQout = 0;
-                    this.turnOffAllMachines();
+                    await this.turnOffAllMachines();
                     return;
                 } else if (demandQ < this.absoluteTotals.flow.min) {
                     this.logger.warn(`Flow demand ${demandQ} is below minimum possible flow ${this.absoluteTotals.flow.min}. Capping to minimum flow.`);
@@ -1184,7 +1181,7 @@ class MachineGroup {
                     this.logger.debug(`Turning machines off`);
                     demandQout = 0;
                     //return early and turn all machines off
-                    this.turnOffAllMachines();
+                    await this.turnOffAllMachines();
                     return;
                 }
                 else{
@@ -1233,6 +1230,12 @@ class MachineGroup {
         await Promise.all(Object.entries(this.machines).map(async ([machineId, machine]) => {
             if (this.isMachineActive(machineId)) { await machine.handleInput("parent", "execsequence", "shutdown"); }
         }));
+        // Update measurements to zero so the parent (PS) sees the
+        // outflow drop immediately — without this the PS keeps the
+        // last active flow value cached and computes wrong net flow.
+        this._writeMeasurement("flow", "predicted", POSITIONS.DOWNSTREAM, 0, this.unitPolicy.canonical.flow);
+        this._writeMeasurement("flow", "predicted", POSITIONS.AT_EQUIPMENT, 0, this.unitPolicy.canonical.flow);
+        this._writeMeasurement("power", "predicted", POSITIONS.AT_EQUIPMENT, 0, this.unitPolicy.canonical.power);
     }
 
     _buildUnitPolicy(config = {}) {