Stabilize Forecast viewer lifecycle

Reuse a single Perspective worker across version switches and delete
the previous table instead of terminating the worker — terminate was
returning a rejecting promise the sync try/catch missed, and each new
worker leaked WASM memory. applyLayout no longer leaks a view per call;
it reads schema directly from the table. An init id guards against
concurrent runs (StrictMode, rapid version switches) clobbering each
other, and a catch on "already exists" recovers via open_table+delete
when a stale table from a previous run is still hosted.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -71,6 +71,7 @@ export default function Forecast({ sourceId, versionId }) {
   const tableRef       = useRef(null)
   const colMetaRef     = useRef([])
   const expandDepthRef = useRef(null)
+  const initIdRef      = useRef(0)
 
   function onDragStart(e) {
     e.preventDefault()
@@ -136,6 +137,7 @@ export default function Forecast({ sourceId, versionId }) {
   async function initViewer(vid, sid) {
     const viewer = viewerRef.current
     if (!viewer) return
+    const myId = ++initIdRef.current
     setLoading(true)
     setLargeDataset(false)
     setLoadProgress(null)
@@ -177,12 +179,37 @@ export default function Forecast({ sourceId, versionId }) {
 
       if (rowCount >= 500000) setLargeDataset(true)
 
-      if (workerRef.current) { try { workerRef.current.terminate() } catch {} }
-      const worker = await perspective.worker()
-      workerRef.current = worker
-      tableRef.current  = rowCount > 0
-        ? await worker.table(buffer, { name: tableName, index: 'pf_id' })
-        : await worker.table([], { name: tableName, index: 'pf_id' })
+      if (myId !== initIdRef.current) return
+
+      if (!workerRef.current) workerRef.current = await perspective.worker()
+      const worker = workerRef.current
+
+      if (tableRef.current) {
+        try { await tableRef.current.delete() } catch {}
+        tableRef.current = null
+      }
+
+      const opts = { name: tableName, index: 'pf_id' }
+      const makeTable = async () => rowCount > 0 ? worker.table(buffer, opts) : worker.table([], opts)
+      try {
+        tableRef.current = await makeTable()
+      } catch (err) {
+        if (/already exists/i.test(String(err?.message || err))) {
+          try {
+            const existing = await worker.open_table(tableName)
+            if (existing) await existing.delete()
+          } catch {}
+          tableRef.current = await makeTable()
+        } else {
+          throw err
+        }
+      }
+
+      if (myId !== initIdRef.current) {
+        try { await tableRef.current.delete() } catch {}
+        tableRef.current = null
+        return
+      }
 
       await viewer.load(worker)
       viewer.setAttribute('theme', dark ? 'Pro Dark' : 'Pro Light')
@@ -283,7 +310,7 @@ export default function Forecast({ sourceId, versionId }) {
   async function applyLayout(layout) {
     const viewer = viewerRef.current
     if (!viewer) return
-    const validCols = new Set(tableRef.current ? Object.keys(await (await tableRef.current.view()).schema()) : [])
+    const validCols = new Set(tableRef.current ? Object.keys(await tableRef.current.schema()) : [])
     const cfg = cleanLayout(layout.config, validCols)
     await viewer.restore(cfg)
     if (cfg.plugin_config) {