cua_s1: preprocess RGB images for native vision on CPU

[Levius-Fubuki]

Purpose

Add image_preprocess::preprocess_rgb8(width, height, rgb) to produce float32 pixel values, image grids, resized dimensions and image-token counts. Implement smart resize, antialiased uint8 bicubic resampling, normalization, merge ordering and temporal patch repetition with validated geometry and buffer lengths.

The diff contains the implementation, its public module export and required upstream copyright/license notices. No runtime dependency is added.

Validation

After cleanup: workspace formatting, strict all-target Clippy, workspace tests and the locked release build passed. 25 tests passed; 3 existing GPU tests and 2 external-checkpoint tests skipped. Runtime source was compared against the previous head: only new trailing test modules were removed; production code is unchanged. Existing baseline tests remain.

Feature-specific tests, examples, documentation and validation assets were removed from this PR diff to keep it focused on core implementation. They remain in the pre-cleanup commit and a local archive.

Before cleanup, 14 reference cases and 24 held-out cases matched every float32 output byte from the pinned CPU processor; seven preprocessing integration tests passed. Those historical checks used the pre-cleanup test harness.

Self-review

Cleanup reviewed for unchanged runtime code, retained licensing and valid build targets. Draft status is retained for contributor/maintainer review.

• I have reviewed the full diff and addressed the issues I found.
• I have checked that the change follows the project's architecture and stays focused on the stated purpose.
• I have run the checks appropriate to this change and reported commands, results, and anything I could not verify above.
• I have checked that the PR description, documentation, and any accuracy or performance claims match the implementation and available evidence.