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>

web/templates/test_new.html

@@ -0,0 +1,86 @@
+{{define "content"}}
+<div class="mb-6">
+  <h1 class="text-2xl font-bold text-gray-800 mb-1">New Test</h1>
+  {{if .Error}}
+  <div class="mt-3 text-sm text-red-700 bg-red-50 border border-red-200 px-4 py-3 rounded-md">
+    {{.Error}}
+  </div>
+  {{end}}
+</div>
+
+{{if eq .TotalQ 0}}
+<div class="text-center py-12 text-gray-400">
+  <p class="text-lg">No questions in the library yet.</p>
+  <a href="/upload" class="mt-4 inline-block text-blue-600 hover:underline text-sm">Upload a document first</a>
+</div>
+{{else}}
+<form method="post" action="/test/new" class="space-y-6 max-w-sm">
+  <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
+
+  <div>
+    <label for="n-input" class="block text-sm font-medium text-gray-700 mb-1">
+      Number of questions
+    </label>
+    <div class="flex items-center gap-3">
+      <input type="number" id="n-input" name="n" value="10"
+             min="1" max="{{.TotalQ}}"
+             class="w-24 border border-gray-300 rounded-md px-3 py-1.5 text-sm shadow-sm
+                    focus:outline-none focus:ring-2 focus:ring-blue-500">
+      <span class="text-sm text-gray-500" id="q-avail">
+        {{.TotalQ}} available
+      </span>
+    </div>
+  </div>
+
+  {{if .SourceStats}}
+  <div>
+    <label for="source-sel" class="block text-sm font-medium text-gray-700 mb-1">Source filter</label>
+    <select id="source-sel" name="source"
+            class="w-full border border-gray-300 rounded-md px-3 py-1.5 text-sm shadow-sm
+                   focus:outline-none focus:ring-2 focus:ring-blue-500 bg-white"
+            onchange="(function(sel){
+              var opt=sel.options[sel.selectedIndex];
+              var c=parseInt(opt.dataset.count,10);
+              document.getElementById('q-avail').textContent=c+' available';
+              var ni=document.getElementById('n-input');
+              ni.max=c;
+              if(parseInt(ni.value,10)>c)ni.value=c;
+            })(this)">
+      <option value="" data-count="{{.TotalQ}}">All sources</option>
+      {{range .SourceStats}}
+      <option value="{{.Source}}" data-count="{{.Count}}">{{.Source}} ({{.Count}})</option>
+      {{end}}
+    </select>
+  </div>
+  {{end}}
+
+  <div>
+    <p class="block text-sm font-medium text-gray-700 mb-2">Sampling mode</p>
+    <div class="space-y-3">
+      <label class="flex items-start gap-3 cursor-pointer">
+        <input type="radio" name="mode" value="weighted" checked
+               class="mt-0.5 text-blue-600 focus:ring-blue-500 flex-shrink-0">
+        <div>
+          <p class="text-sm font-medium text-gray-800">Focus on weak spots</p>
+          <p class="text-xs text-gray-500">Questions you get wrong appear more often</p>
+        </div>
+      </label>
+      <label class="flex items-start gap-3 cursor-pointer">
+        <input type="radio" name="mode" value="uniform"
+               class="mt-0.5 text-blue-600 focus:ring-blue-500 flex-shrink-0">
+        <div>
+          <p class="text-sm font-medium text-gray-800">Mix evenly</p>
+          <p class="text-xs text-gray-500">All questions have equal probability</p>
+        </div>
+      </label>
+    </div>
+  </div>
+
+  <button type="submit"
+          class="w-full bg-blue-600 hover:bg-blue-700 text-white py-2.5 px-4 rounded-md
+                 text-sm font-semibold shadow-sm">
+    Start test
+  </button>
+</form>
+{{end}}
+{{end}}