fix(ios): weight resolution pixel-distance so targetResolution isn't overridden by FPS

[richkuo]

Summary

Fixes a case where targetResolution is effectively ignored when a higher FPS is requested: on an iPhone front TrueDepth camera, asking for 1080p @ 60fps records 3840×2160@60 instead of the device's native 1920×1080@60 format.

Addresses #3963.

Root cause

In CMVideoDimensions.penalty(_:), pixel-distance is the raw log-ratio of pixel counts (range ~0–1.4), while the FPS penalty (AVFrameRateRange.penalty(for:)) is in raw frame units (max(undershoot, overshoot), up to 30) and carries a higher weight in ConstraintResolver's weighted sum. So resolution distance is a very weak signal — when the requested FPS isn't available at the target resolution, the resolver escalates resolution to satisfy FPS.

Per-format penalties at target 1080p / 60fps on a front TrueDepth camera (lower = selected):

format @60fps	penalty
1920×1080 @60	54.5
1280×720 @60	62.6
3840×2160 @60	9.7 ← selected

The resolutionBias term actually favors 1080p (its logPixelDistance is 0; 4K's is 1.386), but it's swamped by the other constraints.

Change

Scale logPixelDistance by the existing aspectMismatchWeight so pixel-distance is weighted comparably to the aspect-ratio term — keeping the requested resolution a first-class signal. This is intentionally local to the resolution penalty and does not change FPS tie-breaking among equally-close resolutions.

let logPixelDistance = aspectMismatchWeight * Swift.abs(log(actualPixels / targetPixels))

Notes

• Marked draft — I'm happy to take a different direction if you'd prefer, e.g. normalizing the FPS penalty to a relative scale instead of strengthening the resolution term. Either resolves the incommensurable-scales issue; this PR takes the smaller, more localized route.
• Verified on-device: with this change, a front-camera 1k@60 request records native 1920×1080@60 (no escalation, no re-encode); 4k mode is unaffected.

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LLM: Claude Opus 4.8 (1M) | Effort: High | Harness: Claude Code

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