fix(editor): reject partially numeric opacity buffers

Add /^\d+$/ guard to opacityFromBuffer so strings like '12x', '5abc',
or '3.5' return 100% instead of silently parsing the numeric prefix.
Add unit tests for partial, decimal, scientific, negative, and
whitespace-prefixed buffers.

packages/core/src/editor/nodes.ts

@@ -10,6 +10,7 @@ import { createVariableBindingActions } from './variable-bindings'
 
 export function opacityFromBuffer(buffer: string): number {
   if (buffer === '0') return 1
+  if (!/^\d+$/.test(buffer)) return 1
   const n = Number.parseInt(buffer, 10)
   if (!Number.isFinite(n)) return 1
   const percent = buffer.length === 1 ? n * 10 : n

tests/engine/editor/opacity.test.ts

@@ -150,4 +150,13 @@ describe('opacityFromBuffer', () => {
     expect(opacityFromBuffer('')).toBe(1)
     expect(opacityFromBuffer('abc')).toBe(1)
   })
+
+  test('rejects partially numeric buffers', () => {
+    expect(opacityFromBuffer('12x')).toBe(1)
+    expect(opacityFromBuffer('5abc')).toBe(1)
+    expect(opacityFromBuffer('3.5')).toBe(1)
+    expect(opacityFromBuffer('1e2')).toBe(1)
+    expect(opacityFromBuffer('-5')).toBe(1)
+    expect(opacityFromBuffer(' 5')).toBe(1)
+  })
 })