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/nodeClass.js

@@ -271,6 +271,7 @@ class nodeClass {
     this.node.on("close", (done) => {
       clearInterval(this._tickInterval);
       clearInterval(this._statusInterval);
+      this.node.status({});  // clear node status badge
       if (typeof done === 'function') done();
     });
   }