fix: text matching Object.prototype property names silently not rendered

Fixes #746.

`graphemeImages` was created as a plain object via `{ ...options.graphemeImages }`,
which means it inherits from `Object.prototype`. When the text being rendered
exactly matches an inherited property name (e.g. "constructor", "toString",
"valueOf", "hasOwnProperty"), the lookup `graphemeImages[text]` returned the
inherited method instead of `undefined`. Downstream code then treated that
function as an image URL and emitted `<image href="function Object() { [native
code] }" ...>` instead of rendering the text as glyph paths.

The fix changes the backing storage to a null-prototype object via
`Object.assign(Object.create(null), options.graphemeImages)`. Bracket lookups
for non-own properties now return `undefined` as expected, and the text falls
through to the normal glyph-rendering path. No behaviour change for legitimate
emoji/image keys.

Test plan: new case in `test/emoji.test.tsx` that renders the text
"constructor toString valueOf" without any graphemeImages and asserts the SVG
contains no `[native code]`, `[object Object]`, or `<image>` element. All
existing emoji and basic tests continue to pass.

Author: Vitalini

src/satori.ts

@@ -73,7 +73,14 @@ export default async function satori(
   root.setJustifyContent(Yoga.JUSTIFY_FLEX_START)
   root.setOverflow(Yoga.OVERFLOW_HIDDEN)
 
-  const graphemeImages = { ...options.graphemeImages }
+  // Null-prototype object so that `graphemeImages[text]` lookups for strings
+  // that match `Object.prototype` property names (e.g. "constructor",
+  // "toString", "valueOf") return `undefined` instead of inherited methods.
+  // See https://github.com/vercel/satori/issues/746.
+  const graphemeImages: Record<string, string> = Object.assign(
+    Object.create(null),
+    options.graphemeImages
+  )
   // Some Chinese characters have different glyphs in Chinese and
   // Japanese, but their Unicode is the same. If the user needs to display
   // the Chinese and Japanese characters simultaneously correctly, the user

test/emoji.test.tsx

@@ -116,4 +116,18 @@ describe('Emojis', () => {
 
     expect(await toImage(svg)).toMatchImageSnapshot()
   })
+
+  // https://github.com/vercel/satori/issues/746
+  it('should render text that matches Object.prototype property names', async () => {
+    const svg = await satori(<div>constructor toString valueOf</div>, {
+      width: 200,
+      height: 100,
+      fonts,
+    })
+    // The text used to be silently replaced with the inherited prototype
+    // method, e.g. `<image href="function Object() { [native code] }" .../>`.
+    expect(svg).not.toContain('[native code]')
+    expect(svg).not.toContain('[object Object]')
+    expect(svg).not.toContain('<image ')
+  })
 })