fix: don't poison shard cache on failure; require real draws for ready status

Two Important review findings on useDrawDistribution:
- ensureShardRegistered cached the rejected promise on fetch/registration
  failure, permanently stuck for that (model,year,state) key until a full
  page reload. Now deletes the cache entry on failure so the next caller
  retries fresh.
- status could become 'ready' even when the LEAID-bound query returned zero
  usable rows (district not present in the draws shard), silently mislabeling
  a fitSkewedInterval-only render as real posterior draws. Now requires at
  least one group with actual draws before reporting 'ready'.

src/hooks/useDrawDistribution.js

@@ -19,13 +19,21 @@ function ensureShardRegistered(db, model, year, state) {
     shardCache.set(
       key,
       (async () => {
-        const url = `${HF_BASE}/model_id=${model}/YEAR=${year}/LEA_STATE=${state}/data_0.parquet`
-        const res = await fetch(url)
-        if (!res.ok) throw new Error(`Failed to fetch draw shard: HTTP ${res.status}`)
-        const buffer = new Uint8Array(await res.arrayBuffer())
-        const fileName = `${key}.parquet`
-        await db.registerFileBuffer(fileName, buffer)
-        return fileName
+        try {
+          const url = `${HF_BASE}/model_id=${model}/YEAR=${year}/LEA_STATE=${state}/data_0.parquet`
+          const res = await fetch(url)
+          if (!res.ok) throw new Error(`Failed to fetch draw shard: HTTP ${res.status}`)
+          const buffer = new Uint8Array(await res.arrayBuffer())
+          const fileName = `${key}.parquet`
+          await db.registerFileBuffer(fileName, buffer)
+          return fileName
+        } catch (err) {
+          // Don't let a transient failure (network blip, HF outage) poison the
+          // cache forever — remove the rejected entry so the next caller for
+          // this shard gets a fresh attempt instead of the same dead promise.
+          shardCache.delete(key)
+          throw err
+        }
       })(),
     )
   }
@@ -79,9 +87,22 @@ export function useDrawDistribution({ leaid, state, model, year, groups }) {
           ;(byGroup[key] ??= []).push(rate)
         }
 
+        // A successful query with zero matching rows (this district isn't in
+        // the draws shard, or none of its rows matched a known group) is not
+        // "ready" — there's no real data to show, so treat it like a failure
+        // and let the caller fall back, rather than silently claiming real
+        // draws while every group actually uses the fitted approximation.
+        const hasDraws = Object.values(byGroup).some((draws) => draws.length > 0)
+
         if (!cancelled) {
-          setDrawsByGroup(byGroup)
-          setStatus('ready')
+          if (hasDraws) {
+            setDrawsByGroup(byGroup)
+            setStatus('ready')
+          } else {
+            console.warn('useDrawDistribution: query succeeded but returned no usable draws for', { leaid, model, year, state })
+            setDrawsByGroup(null)
+            setStatus('error')
+          }
         }
       } catch (err) {
         console.error('useDrawDistribution failed:', err)