feat(config): apply fixed_size ±3 as stabilized defaults

Why:
- Final benchmark selected fixed_size with neighbor expansion ±3 as the winning Candidate.

Changes:
- Set neighbor_prev/neighbor_next defaults to 3 in Settings.
- Default Query tab to fixed_size ±3/3 and Benchmarks to single-strategy fixed_size.

Impact:
- API and Dashboard runs use ±3 unless explicitly overridden.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/static/index.html

@@ -1287,8 +1287,8 @@ function RetrievalInspectView({ experiment, onBack }) {
 
 function QueryTab({ documents, strategies, addToast, formatFilter = 'word', hideTitle = false }) {
   const [form, setForm] = useState({
-    document_id: '', strategy: '', question: '', top_k: 5,
-    neighbor_prev: 0, neighbor_next: 0, corpus_model_id: '',
+    document_id: '', strategy: 'fixed_size', question: '', top_k: 5,
+    neighbor_prev: 3, neighbor_next: 3, corpus_model_id: '',
   });
   const [result, setResult] = useState(null);
   const [loading, setLoading] = useState(false);
@@ -1761,9 +1761,9 @@ function ComparisonView({ experiments, addToast, onBack }) {
 function BenchmarksTab({ documents, strategies, addToast, questionsFile, formatFilter = 'word', hideTitle = false }) {
   const [experiments, setExperiments] = useState([]);
   const [form, setForm] = useState({
-    document_id: '', strategies: strategies.map(s => s.name),
+    document_id: '', strategies: ['fixed_size'],
     questions_file: questionsFile, top_k: 5,
-    neighbor_prev: 0, neighbor_next: 0, dry_run: false, corpus_model_id: '',
+    neighbor_prev: 3, neighbor_next: 3, dry_run: false, corpus_model_id: '',
   });
   const [running, setRunning] = useState(false);
   const [embModels, setEmbModels] = useState([]);