fix: use extra channel's own bit_depth for modular-to-f32 conversion

[lilith]

Summary

Extra channels (alpha, depth, etc.) can have different bit depths than the main color channels. For example, an image might be 8-bit RGB with a 16-bit alpha channel.

Previously, the render pipeline used the image's global metadata.bit_depth for converting all extra channels from modular to f32. This caused incorrect conversion for extra channels with different bit depths.

This PR:

• Adds a bit_depth() getter to ExtraChannelInfo
• Uses each extra channel's own bit depth in ConvertModularToF32Stage

Test plan

• Added unit tests for extra channel bit depth accessor
• All 29 extra channel and alpha-related tests pass
• Note: 3 pre-existing test failures on main (RCT overflow) are unrelated and will be fixed by fix: use wrapping arithmetic for RCT transforms to prevent overflow panic #603

🤖 Generated with Claude Code

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[lilith]

⚠️ Test Limitation

No true failing integration test. A proper regression test would require a JXL file where the image bit depth differs from an extra channel bit depth. Without such a test asset, we can only test the components in isolation.

The added tests demonstrate the math of the bug (using wrong scale produces 257x error), but do not exercise the specific buggy line in frame/render.rs end-to-end.

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Changes in latest push:

• Added test_modular_to_f32_bit_depth_matters showing 257x error from wrong scale
• Added modular_to_f32_8bit_consistency and modular_to_f32_16bit_consistency stage tests
• Removed Claude co-author attribution

[veluca93]

Please remove this comment.

[veluca93]

I am not sure I see the point of all the other tests, except these last two.
Do you mind removing them?