refactor(units): route all conversions through UnitPolicy.convert

Delete the legacy _convertUnitValue helper on the domain and the
duplicate convertUnitValue export on curveNormalizer; both were
identical to UnitPolicy.convert. Callers in flowController, the
curve normalizer, and buildQHCurve now go through this.unitPolicy.
The contract in .claude/refactor/CONTRACTS.md §6 named these as the
target migration; this finishes the rollout for rotatingMachine.

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

src/specificClass.js

@@ -247,12 +247,6 @@ class Machine extends BaseDomain {
     if (!this.isUnitValidForType(type, u)) throw new Error(`Unsupported unit '${u}' for ${type} measurement.`);
     return u;
   }
-  _convertUnitValue(value, from, to, ctx = 'unit conversion') {
-    const n = Number(value);
-    if (!Number.isFinite(n)) throw new Error(`${ctx}: value '${value}' is not finite`);
-    if (!from || !to || from === to) return n;
-    return convert(n).from(from).to(to);
-  }
   _measurementPositionForMetric(metricId) { return metricId === 'power' ? 'atEquipment' : 'downstream'; }
   _resolveProcessRangeForMetric(metricId, predicted, measured) {
     let processMin = NaN; let processMax = NaN;