Phase 6: take a test (weighted sampling + question flow)

- internal/sampling: ComputeWeight (Laplace-smoothed error rate + recency
  multiplier, floor 0.15) and SelectWeighted (A-Res reservoir algorithm).
  10k-run statistical test verifies weak questions appear >3x more often
  than mastered, and mastered questions still appear (floor exercised).
- GET/POST /test/new: source filter with live available-count JS update,
  n-questions input, weighted vs uniform mode radio.
- GET /test/{id}/q/{n}: deterministic answer shuffle per (test_id,
  question_id), progress bar, mobile-friendly large tap targets.
- POST /test/{id}/q/{n}: records answer + upserts stat; advances to next
  question or finishes test and redirects to results stub.
- GET /test/{id}/results: stub (Phase 7 will add full review).
- Ownership enforced: all test routes 404 for wrong user.

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

cmd/server/main.go

@@ -59,6 +59,7 @@ func main() {
 	homeH := handlers.NewHomeHandler(authMgr, repo, renderer)
 	uploadH := handlers.NewUploadHandler(authMgr, repo, llmClient, renderer, cfg.DataDir)
 	questionH := handlers.NewQuestionHandler(authMgr, repo, renderer)
+	testH := handlers.NewTestHandler(authMgr, repo, renderer)
 
 	r := chi.NewRouter()
 	r.Use(middleware.RequestID)
@@ -89,6 +90,11 @@ func main() {
 		r.Get("/questions/{id}", questionH.Show)
 		r.Post("/questions/{id}", questionH.Edit)
 		r.Post("/questions/{id}/delete", questionH.Delete)
+		r.Get("/test/new", testH.NewGet)
+		r.Post("/test/new", testH.NewPost)
+		r.Get("/test/{id}/q/{n}", testH.QuestionGet)
+		r.Post("/test/{id}/q/{n}", testH.QuestionPost)
+		r.Get("/test/{id}/results", testH.ResultsGet)
 	})
 
 	srv := &http.Server{