test: add 182 new tests for router, sections, renderer, and validator

Generated by wave test-gen pipeline. Coverage:
- router.js: 0% → ~85% (33 tests, all 7 exports)
- sections.js: 0% → ~90% (29 tests, all 5 exports)
- renderer.js: partial → extended (36 tests, difficulty, feedback, sidebar)
- validator.js: partial → extended (84 tests, all types + edge cases)

Total: 43 → 225 tests

.gitignore

@@ -13,3 +13,9 @@ coverage
 # Auto-Claude
 .auto-claude
 .worktrees
+
+# Wave ephemeral data
+.wave/workspaces
+.wave/traces
+.wave/artifacts
+.wave/output