[tmathern]

Description

• Reduces Rust's watermarking memory use by reusing model input and avoiding full-image copies.
• Adds empty-image handling (fixes a potential panic).

Related Issue

N/A

Motivation and Context

Reducing memory usage.

How Has This Been Tested?

• (Temporary) probes ensuring watermark parity between this branch and main.
• Unit tests.

Screenshots (if appropriate): N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.