Predicted-volume hard-floor at 0 + spill flow position refactor

Volume integrator changes:
- Hard physical floor at 0 added to _updatePredictedVolume. Without
  it, a basin seeded below dryRunSafetyVol (calibration / startup
  / low seed) under continued net-outflow drifted volume arbitrarily
  negative; the level output looked clamped only because
  _calcLevelFromVolume floors at 0, masking the underlying drift.
- New cumulative diagnostic: underflowVolume.predicted.atequipment
  (m³) + getOutput().predictedUnderflowVolume. Non-zero indicates a
  flow-balance error (over-reported outflow / missing inflow).
- The transition-only dryRunSafetyVol clamp is preserved so
  startup-from-empty doesn't snap to 2.1 m³ on tick 1.

Spill flow refactor (taxonomic + bug fix):
- Synthetic spill moved from flow.predicted.out.<child='overflow'>
  to its own position flow.predicted.overflow.<default>. The spill
  is a derived quantity, not a physical sub-source sharing a position
  with pumps — .child() was the wrong knob.
- Removes the spillPrev self-subtraction in the integrator (no longer
  needed: outflowTotal at ['out','downstream'] cleanly excludes spill).
- Closes a latent fall-through bug exposed during this work:
  .child('overflow').getCurrentValue() returned the value of any
  available sibling child when overflow itself didn't yet exist.
  Hardened separately in generalFunctions@a516c2b.
- _selectBestNetFlow folds the overflow position into the outflow
  side so the predicted net-flow balance still reads ~0 while pinned.

Tests: 70/70 pass. 4 new subtests cover the 0-floor, accumulated
underflow tracking, getOutput surface, and refill-from-empty.

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

test/basic/specificClass.test.js

@@ -472,7 +472,7 @@ test('Predicted volume — overflow clamp and spill tracking', async (t) => {
     assert.equal(vol, 45);  // pinned at overflow
     const cumulative = ps.measurements.type('overflowVolume').variant('predicted').position('atequipment').getCurrentValue('m3');
     assert.equal(cumulative, 1);  // proposed=44+2=46, excess=1 m³ this tick
-    const spill = ps.measurements.type('flow').variant('predicted').position('out').child('overflow').getCurrentValue('m3/s');
+    const spill = ps.measurements.type('flow').variant('predicted').position('overflow').getCurrentValue('m3/s');
     assert.equal(spill, 2);  // instantaneous balance: inflow − outflowReal
   });
 
@@ -483,7 +483,7 @@ test('Predicted volume — overflow clamp and spill tracking', async (t) => {
     assert.equal(vol, 45);
     const cumulative = ps.measurements.type('overflowVolume').variant('predicted').position('atequipment').getCurrentValue('m3');
     assert.equal(cumulative, 3);  // 1 + 2
-    const spill = ps.measurements.type('flow').variant('predicted').position('out').child('overflow').getCurrentValue('m3/s');
+    const spill = ps.measurements.type('flow').variant('predicted').position('overflow').getCurrentValue('m3/s');
     assert.equal(spill, 2);
   });
 
@@ -499,7 +499,7 @@ test('Predicted volume — overflow clamp and spill tracking', async (t) => {
     ps._predictedFlowState = { inflow: 0, outflow: 0, lastTimestamp: t0 + 2000 };
     Date.now = () => t0 + 3000;
     ps._updatePredictedVolume();
-    const spill = ps.measurements.type('flow').variant('predicted').position('out').child('overflow').getCurrentValue('m3/s');
+    const spill = ps.measurements.type('flow').variant('predicted').position('overflow').getCurrentValue('m3/s');
     assert.equal(spill, 0);
     // Volume stays at 45 (no draining force) but is no longer "pinned".
     const vol = ps.measurements.type('volume').variant('predicted').position('atequipment').getCurrentValue('m3');
@@ -549,3 +549,53 @@ test('getOutput — exposes predictedOverflowVolume / predictedOverflowRate', ()
   assert.equal(out.predictedOverflowVolume, 1);
   assert.equal(out.predictedOverflowRate, 2);
 });
+
+// Hard physical floor at 0. The dryRunSafetyVol clamp only fires on transition
+// from above, so a basin seeded below + continued outflow used to integrate
+// the volume arbitrarily negative. The level helper masked this by flooring
+// at 0 in _calcLevelFromVolume — fix is to floor the integrator itself.
+test('Predicted volume — physical floor at 0 (underflow track)', async (t) => {
+  const ps = new PumpingStation(makeConfig({
+    safety: { enableDryRunProtection: true, dryRunThresholdPercent: 5 },
+  }));
+  const t0 = 1_700_000_000_000;
+
+  await t.test('seeded below dryRun + continued outflow does NOT go negative', () => {
+    ps.calibratePredictedVolume(0.5, t0);  // below dryRunSafetyVol (2.1)
+    ps.setManualOutflow(2, t0, 'm3/s');     // 2 m³/s for 1s → would drop to -1.5
+    ps._predictedFlowState = { inflow: 0, outflow: 2, lastTimestamp: t0 };
+    Date.now = () => t0 + 1000;
+    ps._updatePredictedVolume();
+    const vol = ps.measurements.type('volume').variant('predicted').position('atequipment').getCurrentValue('m3');
+    assert.equal(vol, 0);  // floored at 0, not -1.5
+    const underflow = ps.measurements
+      .type('underflowVolume').variant('predicted').position('atequipment').getCurrentValue('m3');
+    assert.equal(underflow, 1.5);  // tracked as diagnostic
+  });
+
+  await t.test('subsequent ticks accumulate underflow while outflow continues', () => {
+    Date.now = () => t0 + 2000;
+    ps._predictedFlowState = { inflow: 0, outflow: 2, lastTimestamp: t0 + 1000 };
+    ps._updatePredictedVolume();
+    const vol = ps.measurements.type('volume').variant('predicted').position('atequipment').getCurrentValue('m3');
+    assert.equal(vol, 0);
+    const underflow = ps.measurements
+      .type('underflowVolume').variant('predicted').position('atequipment').getCurrentValue('m3');
+    assert.equal(underflow, 3.5);  // 1.5 + 2.0
+  });
+
+  await t.test('getOutput exposes predictedUnderflowVolume', () => {
+    const out = ps.getOutput();
+    assert.equal(out.predictedUnderflowVolume, 3.5);
+  });
+
+  await t.test('inflow returns and basin refills from 0 (no jump to dryRunSafetyVol)', () => {
+    ps.setManualInflow(1, t0 + 2000, 'm3/s');
+    ps.setManualOutflow(0, t0 + 2000, 'm3/s');
+    ps._predictedFlowState = { inflow: 1, outflow: 0, lastTimestamp: t0 + 2000 };
+    Date.now = () => t0 + 3000;
+    ps._updatePredictedVolume();
+    const vol = ps.measurements.type('volume').variant('predicted').position('atequipment').getCurrentValue('m3');
+    assert.ok(Math.abs(vol - 1) < 1e-9);  // 0 + 1 = 1, NOT pinned to 2.1
+  });
+});