Replace sibling attributes on a key change, and say when none were found

Autofill filled only empty boxes, so it appeared to work exactly once. Type a
part, get its nine attributes; type a different part, and every box is
already full, so nothing updates and the form still describes the previous
part. That is worse than blank -- the recode would have been submitted with
one part's code and another's attributes.

These columns describe the key that was just entered, so they are replaced
outright now.

A lookup that finds nothing also said nothing, which is indistinguishable
from a lookup that did not run. It warns, and names the value it could not
find, so an unknown part reads differently from a broken autofill.

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

ui/src/views/Forecast.jsx

@@ -1098,12 +1098,18 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       if (!res.ok) return
       derived = await res.json()
     }
-    if (!derived) return
+    if (!derived) {
+      flash(`No attributes found for ${col} "${value.trim()}" — the other fields are unchanged`, 'warn')
+      return
+    }
+
+    // Overwrite rather than only filling blanks. These columns describe the key
+    // that was just entered, so whatever a previous key left behind is wrong, not
+    // worth preserving -- and leaving it made a second lookup look like it had
+    // done nothing at all, since every box was already full.
     setter(prev => {
       const next = { ...prev }
-      for (const [k, v] of Object.entries(derived)) {
-        if (!prev[k] || prev[k] === '') next[k] = String(v ?? '')
-      }
+      for (const [k, v] of Object.entries(derived)) next[k] = String(v ?? '')
       return next
     })
   }