Phase 1 wave 1: domain + nodered + stats infra (additive)

Adds platform infrastructure used by the upcoming refactor of
nodeClass / specificClass across all 12 nodes:

- src/domain/UnitPolicy.js     — extracted from rotatingMachine/MGC
- src/domain/ChildRouter.js    — declarative event routing on top of childRegistrationUtils
- src/domain/LatestWinsGate.js — extracted from MGC dispatch gate
- src/domain/HealthStatus.js   — standardised {level, flags, message, source}
- src/nodered/statusBadge.js   — compose / error / idle / byState / text helpers
- src/stats/index.js           — mean / stdDev / median / mad / lerp

All additive — no existing exports change shape.
56 unit tests pass under node:test.

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

src/nodered/statusBadge.js

@@ -0,0 +1,96 @@
+/**
+ * statusBadge — small helpers that build Node-RED status objects
+ * ({ fill, shape, text }) consistently across every node.
+ *
+ * See CONTRACTS.md §7. Domains compose badges via these helpers so the
+ * editor look-and-feel converges instead of every node rolling its own
+ * emoji + colour rules.
+ */
+
+'use strict';
+
+const MAX_TEXT = 60;
+const SEPARATOR = ' | ';
+
+const DEFAULT_BADGE = { fill: 'green', shape: 'dot' };
+const ERROR_BADGE   = { fill: 'red',   shape: 'ring' };
+const IDLE_BADGE    = { fill: 'blue',  shape: 'dot' };
+const UNKNOWN_BADGE = { fill: 'grey',  shape: 'ring' };
+
+// Truncate to MAX_TEXT keeping room for the ellipsis. Editor clips the
+// rest visually anyway, but we want the cut to be deterministic so
+// snapshot tests don't drift across Node-RED versions.
+function _clip(text) {
+    if (text == null) return '';
+    const s = String(text);
+    if (s.length <= MAX_TEXT) return s;
+    return s.slice(0, MAX_TEXT - 1) + '…';
+}
+
+function _joinParts(parts) {
+    if (!Array.isArray(parts) || parts.length === 0) return '';
+    const kept = parts.filter((p) => p != null && p !== false && p !== '');
+    if (kept.length === 0) return '';
+    return kept.map(String).join(SEPARATOR);
+}
+
+function compose(parts, opts) {
+    const text = _clip(_joinParts(parts));
+    return {
+        fill:  (opts && opts.fill)  || DEFAULT_BADGE.fill,
+        shape: (opts && opts.shape) || DEFAULT_BADGE.shape,
+        text,
+    };
+}
+
+function error(message) {
+    return {
+        fill:  ERROR_BADGE.fill,
+        shape: ERROR_BADGE.shape,
+        text:  _clip(`⚠ ${message == null ? '' : message}`),
+    };
+}
+
+function idle(label) {
+    return {
+        fill:  IDLE_BADGE.fill,
+        shape: IDLE_BADGE.shape,
+        text:  _clip(`⏸️ ${label == null ? '' : label}`),
+    };
+}
+
+// Look up a state-template badge and optionally compose extra parts
+// into its text. Missing template falls back to a grey "unknown state"
+// badge — silent so caller can still surface the bad state through logs.
+function byState(stateMap, currentState, opts) {
+    const template = stateMap && stateMap[currentState];
+    if (!template) {
+        return {
+            fill:  UNKNOWN_BADGE.fill,
+            shape: UNKNOWN_BADGE.shape,
+            text:  _clip(`unknown state: ${currentState == null ? '' : currentState}`),
+        };
+    }
+    const baseText = template.text == null ? '' : String(template.text);
+    const extras   = opts && Array.isArray(opts.compose) ? opts.compose : [];
+    const merged   = extras.length > 0
+        ? _joinParts([baseText, ...extras])
+        : baseText;
+    return {
+        fill:  template.fill  || DEFAULT_BADGE.fill,
+        shape: template.shape || DEFAULT_BADGE.shape,
+        text:  _clip(merged),
+    };
+}
+
+function text(string, opts) {
+    return {
+        fill:  (opts && opts.fill)  || DEFAULT_BADGE.fill,
+        shape: (opts && opts.shape) || DEFAULT_BADGE.shape,
+        text:  _clip(string == null ? '' : string),
+    };
+}
+
+const statusBadge = { compose, error, idle, byState, text };
+
+module.exports = { statusBadge, MAX_TEXT };