Restructure test files to src/*.test.ts format

Move Group C (C1-C3 balance-aware fallback) and Group E (E1-E2 edge cases)
tests from scripts/ to src/*.test.ts format, following the Group B pattern.

- New: balance-selection-fallback.test.ts (4 tests: C1, C2, C3, C+ bonus)
- New: balance-selection-edge.test.ts (2 tests: E1, E2)
- Removed: scripts/balance-aware-tests.ts
- Removed: scripts/balance-aware-tests-e.ts
- Updated: package.json with new test scripts

Author: rictus

e2e/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "test": "tsx test.ts",
     "test:unit": "tsx src/balance-aware-selector.test.ts",
-    "test:balance-e": "tsx scripts/balance-aware-tests-e.ts",
+    "test:balance-c": "tsx src/balance-selection-fallback.test.ts",
+    "test:balance-e": "tsx src/balance-selection-edge.test.ts",
     "test:watch": "tsc --watch",
     "setup": "./setup.sh",
     "setup:legacy": "./setup.sh --legacy",