feat(middleware): add datapoint transform engine with round, scale, and map support

- src/modules/datapoints/transforms.ts: applyTransform() supporting map (priority), scale, and round
- tests/transforms.test.ts: 17 tests covering no-transform passthrough, round precision, scale multiply, map lookup, map priority over numeric transforms, combined transforms, and edge cases

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

display/middleware/src/modules/datapoints/transforms.ts

@@ -0,0 +1,45 @@
+export interface TransformConfig {
+  round?: number;
+  scale?: number;
+  map?: Record<string, string>;
+}
+
+/**
+ * Apply a transformation pipeline to a raw string value.
+ *
+ * Processing order:
+ * 1. If `map` is defined and the value exists as a key, return the mapped value immediately.
+ * 2. Parse the value as a float. If it's not numeric, return as-is.
+ * 3. Apply `scale` (multiply).
+ * 4. Apply `round` (toFixed, then remove trailing zeros).
+ */
+export function applyTransform(value: string, config: TransformConfig | null): string {
+  if (!config || Object.keys(config).length === 0) {
+    return value;
+  }
+
+  // Map takes priority — if key matches, return immediately without numeric transforms
+  if (config.map && Object.prototype.hasOwnProperty.call(config.map, value)) {
+    return config.map[value];
+  }
+
+  // Only apply numeric transforms if value is actually a number
+  const num = parseFloat(value);
+  if (isNaN(num)) {
+    return value;
+  }
+
+  let result = num;
+
+  // Apply scale
+  if (typeof config.scale === 'number') {
+    result = result * config.scale;
+  }
+
+  // Apply round
+  if (typeof config.round === 'number') {
+    result = parseFloat(result.toFixed(config.round));
+  }
+
+  return String(result);
+}