fix: restore fence tool handlers dropped by null-guard commit

251af2c added a `?.` on cursorRef in onGridClick but inadvertently
deleted the closing braces, the trailing reset block, and the
onKeyDown/onKeyUp/onCancel handlers — leaving onGridClick syntactically
unterminated and three undefined references at the emitter.on/off sites.
Restores the deleted block verbatim, keeping the optional-chaining null
guard on cursorRef.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/src/fence/tool.tsx

@@ -510,6 +510,26 @@ export const FenceTool: React.FC = () => {
         startingPoint.current.set(nextStart[0], event.localPosition[1], nextStart[1])
         endingPoint.current.copy(startingPoint.current)
         cursorRef.current?.position.copy(startingPoint.current)
+        previewRef.current.visible = false
+        buildingState.current = 1
+        setDraftMeasurement(null)
+      }
+    }
+
+    const onKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Shift') shiftPressed.current = true
+    }
+
+    const onKeyUp = (e: KeyboardEvent) => {
+      if (e.key === 'Shift') shiftPressed.current = false
+    }
+
+    const onCancel = () => {
+      if (buildingState.current === 1) {
+        markToolCancelConsumed()
+        stopDrafting()
+      }
+    }
 
     emitter.on('grid:move', onGridMove)
     emitter.on('grid:click', onGridClick)