Name each loaded table uniquely

Refresh Data aborted with Table "fc_29" already exists. The name was fixed
per version and the cleanup before it is best-effort: the viewer is still
holding the previous table when the new one is built, so deleting it does
not free the registry entry, and creating a second under the same name
fails.

Nothing reads the name -- the viewer is loaded by reference, and cleanLayout
strips it out of saved configs -- so it only has to be unique. The previous
name is remembered and freed on the next load, when the viewer has let go of
it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -246,6 +246,9 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
   // the datagrid plugin element, for reading cell coordinates and driving its
   // own selection highlight (see the selection-highlight effect below)
   const gridRef        = useRef(null)
+  // the worker-registry name of the table currently loaded, so the next load can
+  // try to free it
+  const tableNameRef   = useRef(null)
   // { x, y } of the body cell the last mousedown landed on, or null
   const downCellRef    = useRef(null)
   // the region a drag is building, as a Perspective ViewWindow; committed on mouseup
@@ -765,7 +768,13 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
             ...meta.filter(c => ['dimension','value','units','date'].includes(c.role)).map(c => c.cname),
             'pf_id', 'pf_iter', 'pf_logid', 'pf_user', 'created_at', 'pf_segment', 'pf_bucket', 'pf_note', 'pf_op',
           ])
-      const tableName = `fc_${vid}`
+      // Unique per load, not per version. A fixed name collides on refresh:
+      // the viewer still holds the previous table, so deleting it does not
+      // actually free the registry entry, and creating the new one aborts with
+      // "Table `fc_29` already exists". Nothing reads the name -- the viewer is
+      // loaded by reference and cleanLayout strips the name out of saved
+      // configs -- so it only has to be unique.
+      const tableName = `fc_${vid}_${myId}`
 
       if (rowCount >= 500000) setLargeDataset(true)
 
@@ -774,16 +783,21 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       if (!workerRef.current) workerRef.current = await perspective.worker()
       const worker = workerRef.current
 
-      // Clean up the previous table — by JS reference first, then by name in the
-      // worker registry (covers the case where the ref was lost or delete failed).
+      // Clean up the previous table — by JS reference first, then by the name we
+      // gave it (covers the case where the ref was lost or the delete failed).
+      // Best-effort: a table the viewer is still holding will refuse, which is
+      // why the new one no longer reuses its name.
       if (tableRef.current) {
         try { await tableRef.current.delete() } catch {}
         tableRef.current = null
       }
-      try {
-        const stale = await worker.open_table(tableName)
-        if (stale) await stale.delete()
-      } catch {}
+      if (tableNameRef.current && tableNameRef.current !== tableName) {
+        try {
+          const stale = await worker.open_table(tableNameRef.current)
+          if (stale) await stale.delete()
+        } catch {}
+      }
+      tableNameRef.current = tableName
 
       // An empty result gets no index. `[]` carries no columns, so naming one
       // aborts the worker outright -- "Specified index `pf_gkey` does not exist