Fix Picker depth clear decoding

[slimbuck]

Summary

• Fixes Picker#getPointDepthAsync() so RGBA8 depth bits are reconstructed as an unsigned 32-bit value.
• Ensures cleared depth pixels (0xFFFFFFFF) are correctly treated as “no hit” instead of being reinterpreted as NaN.

Details

JavaScript bitwise operations produce signed 32-bit integers. For a cleared depth pixel [255, 255, 255, 255], the previous decode produced -1, so the 0xFFFFFFFF no-depth check failed. This could cause background picks to return a NaN world point.

The fix applies >>> 0 after reconstructing the integer bits.

Test Plan

• Verified background picks return null instead of a NaN world point.
• Verified valid object/gsplat picks still return a world-space point.

[Copilot]

Pull request overview

Fixes depth decoding in Picker#getPointDepthAsync() so RGBA8-packed float bits are reconstructed as an unsigned 32-bit integer, ensuring cleared depth pixels (0xFFFFFFFF) are correctly treated as “no hit” and do not produce NaN world points.

Changes:

• Reconstruct RGBA8 depth bits as an unsigned 32-bit value using >>> 0.
• Restore the intended 0xFFFFFFFF cleared-depth sentinel behavior (returns null).

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