Add automatic highlight rolloff in preprocessing

[disk02]

Motivation

• Overexposed / clipped highlights cause unstable 3DGS predictions and visible artifacts near saturated regions.
• preprocess_one previously only converted uint8->float, divided by 255, and resized, with no highlight handling.
• The goal is an always-on, automatic, lightweight local tone rolloff that only affects extreme highlights and preserves normal images.

Description

• Implemented saturation detection and a soft-knee highlight rolloff inside preprocess_one using only tensor ops on the input device with constants sat_threshold=0.98, sat_frac_threshold=0.001, knee_start=0.98, and rolloff_strength=4.0.
• Rolloff uses the provided formula y = knee_start + (1 - knee_start) * (1 - exp(-k * (x - knee_start) / (1 - knee_start))) and is applied elementwise for x > knee_start, followed by clamp(0,1).
• Added aux metadata keys "highlight_rolloff_applied", "highlight_sat_frac", and "highlight_sat_frac_threshold" so callers can observe when the rolloff ran.
• Added INFO logging when rolloff activates in both the batch CLI loop and the single-image predict_image path.

Testing

• No automated tests were run as part of this change.
• (Manual validation recommended by running the CLI over a mixed set of normal and overexposed images to confirm rolloff triggers only for saturated frames and reduces blown-highlight artifacts.)

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Codex Task