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>

test/basic/flowController.basic.test.js

@@ -27,6 +27,10 @@ function makeHost({
     unitPolicy: {
       canonical: { flow: 'm3/s' },
       output:    { flow: 'm3/h' },
+      convert: (val, from, to, label) => {
+        host.calls.convertUnit.push({ val, from, to, label });
+        return val * 1000; // pretend m3/h -> m3/s factor
+      },
     },
     isValidActionForMode: (action) => allowedActions.has(action),
     isValidSourceForMode: () => allowedSources,
@@ -38,10 +42,6 @@ function makeHost({
       return { moved: sp };
     },
     calcCtrl: (canonicalFlow) => { host.calls.calcCtrl.push(canonicalFlow); return canonicalFlow / 2; },
-    _convertUnitValue: (val, from, to, label) => {
-      host.calls.convertUnit.push({ val, from, to, label });
-      return val * 1000; // pretend m3/h -> m3/s factor
-    },
   };
   return host;
 }