feat: add React image change callback bridge

[Anand-240]

Live previews currently have to poll hasChanges() and getImage() to detect edits. This draft proposes the React side of an image change callback for #87.

The component forwards MountOptions.onChange to the latest onChange prop without remounting, polling, or exporting an image. Notifications from a cancelled mount are ignored. The callback is excluded from options, consistent with the other component callbacks.

Core dependency

The default CDN editor, version 2.13.0, does not emit this callback. This draft needs a corresponding change in the underlying editor before it can replace polling or be released. It does not close #87.

The proposed contract is a notification when the visible image changes. Consumers control snapshot timing and throttling. The core implementation needs to define notifications during brush strokes, undo, redo, adjustments, and resets, and ensure the current image can be read when the callback runs. Tool selection and viewport changes should not trigger it.

Validation

• 76 tests pass with 100% coverage.
• Typecheck and package build pass.
• Lint passes in an isolated copy because a local nested checkout interferes with ESLint configuration discovery in the working folder.
• Demo typecheck and build pass.
• Tests cover callback replacement and removal, no automatic export, and notifications after cancellation or unmount. These use a mocked embed contract; the default CDN cannot verify the event end to end yet.

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[ivoIturrieta]

Thanks for this, and for the clear write-up in #87.

The image editor doesn't have a change event today, so this onChange prop wouldn't fire yet. The wrapper change itself looks good, but we can't merge it until the editor supports the event.

We'll discuss whether to add a change event to the editor and update you here. In the meantime, let's keep this PR open as a draft. Until then, polling hasChanges() and getImage() is the supported approach.