runs filter: persist chip state in URL + server-render initial slice

- runs-filter.js mirrors its chip selections into the query string
  (dataset/algorithm/N/T/J). Empty selections are omitted entirely.
  Reads URL on init; triggers an immediate /runs refresh if any filter
  was present so the polled slice catches up instantly.
- dataset-picker.js's updateUrlState now merges into the existing query
  instead of rebuilding, so the two scripts don't stomp each other's
  keys.
- Index route applies the same chip filter to its initial server-side
  run listing, so a filter-bearing deep-link renders the right slice on
  first paint — no flash of unfiltered runs.

app/web/main.py

@@ -742,14 +742,16 @@ async def index(request: Request) -> HTMLResponse:
     reducers = _reducer_choices()
     default_reducer = reducers[0]["key"] if reducers else None
     default_spec = REDUCERS.get(default_reducer) if default_reducer else None
+    q = request.query_params
+    required = _chip_filter_tags(q)
+    initial_limit = 50 if required else 10
     async with httpx.AsyncClient(timeout=5.0) as client:
-        runs = await PREFECT.recent_runs(client, limit=10)
+        runs = await PREFECT.recent_runs(client, limit=initial_limit, required_tags=required)
         dep_id = await PREFECT.deployment_id(client)
     views = [_run_view(r) for r in runs]
     _mark_stale_views(views)
     # Pre-resolve the two <details> sections' open state from the URL so
     # first paint matches (no flash). Intro defaults closed, picker open.
-    q = request.query_params
     intro_open = q.get("intro") == "1"
     picker_open = q.get("picker") != "0"
     # Also pre-resolve the radio-group selections so n/f/j render with the

app/web/static/dataset-picker.js

@@ -298,7 +298,10 @@ async function main() {
   // the query string so a refresh restores the page. Intro defaults to
   // closed (no param); picker defaults to open (no param).
   function updateUrlState() {
-    const p = new URLSearchParams();
+    // Merge into the current query — other scripts (e.g. runs-filter.js)
+    // also own some keys, so wipe only ours before repopulating.
+    const p = new URLSearchParams(window.location.search);
+    for (const k of ['intro', 'picker', 'ds', 'n', 'f', 'j']) p.delete(k);
     const introEl = document.getElementById('intro');
     const pickerEl = document.getElementById('picker');
     if (introEl && introEl.open) p.set('intro', '1');

app/web/static/runs-filter.js

@@ -37,6 +37,26 @@
     slot.setAttribute('hx-vals', JSON.stringify(stateAsQuery()));
   }
 
+  function syncUrl() {
+    // Mirror chip state to URL query — merge so we don't stomp on other
+    // scripts' keys (dataset-picker writes intro/picker/ds/n/f/j).
+    const p = new URLSearchParams(window.location.search);
+    for (const ax of AXES) p.delete(ax.key);
+    for (const ax of AXES) {
+      if (ax.selected != null) p.set(ax.key, ax.selected);
+    }
+    const qs = p.toString();
+    history.replaceState(null, '', qs ? `?${qs}` : window.location.pathname);
+  }
+
+  function applyUrlState() {
+    const u = new URLSearchParams(window.location.search);
+    for (const ax of AXES) {
+      const v = u.get(ax.key);
+      ax.selected = v || null;
+    }
+  }
+
   function paint(ax) {
     if (!ax.el) return;
     ax.el.innerHTML = '';
@@ -101,6 +121,7 @@
       ax.selected = (ax.selected === v) ? null : v;
       paint(ax);
       syncHtmxVals();
+      syncUrl();
       updateCounter();
       triggerRunsRefresh();
     });
@@ -115,9 +136,15 @@
     }
   });
 
+  applyUrlState();
   syncHtmxVals();
   refreshUniverse();
   updateCounter();
+  // If the URL had filter params, kick a refresh so #runs-slot's initial
+  // HTML (rendered unfiltered server-side) gets replaced with the matching
+  // slice. Without this the user would see the full list until the 3s
+  // poll tick.
+  if (AXES.some((ax) => ax.selected != null)) triggerRunsRefresh();
   // Periodically refresh the universe so newly-introduced values appear.
   setInterval(refreshUniverse, 30_000);
 })();

app/web/templates/index.html

@@ -500,7 +500,7 @@
 <script type="module" src="/static/dataset-picker.js?v=12"></script>
 <script type="module" src="/static/metrics.js?v=11"></script>
 <script src="/static/compare-select.js?v=4"></script>
-<script src="/static/runs-filter.js?v=6"></script>
+<script src="/static/runs-filter.js?v=7"></script>
 <script type="module" src="/static/run-modal.js?v=3"></script>
 <script>
   // Anchor-links alone don't expand <details>; force it.