P10.7a + P10.2: fix test script + remove 5 Mocha-style legacy duplicates

- package.json test script now covers test/ recursively + nrmse/errorMetric.
- Removed 5 broken test files that used Mocha-style describe()/it() globals
  with no test runner installed. All 5 have working kebab-case node:test
  equivalents (e.g. childRegistration.test.js → child-registration-utils.test.js).
- 4 remaining pre-existing assertion failures in output-utils + validation-utils
  logged in OPEN_QUESTIONS.md for Phase 10.5.

166/170 tests pass (4 known pre-existing failures).

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

package.json

@@ -19,7 +19,7 @@
   },
 
   "scripts": {
-    "test": "node --test test/*.test.js src/nrmse/errorMetric.test.js"
+    "test": "node --test test/ src/nrmse/errorMetric.test.js"
   },
   "repository": {
     "type": "git",