bevy_color: Unclamp with_luminance and fix the Laba to Lcha conversion

[stuartparmenter]

Objective

LinearRgba::with_luminance clamps its result to [0.0, 1.0], so it destroys HDR and wide-gamut values, and the clamped result does not have the requested luminance. The Laba to Lcha conversion clamps chroma, which cuts off wide-gamut colors. Several channel docs also claim a [0, 1] range that HDR colors already exceed.

Solution

• Makes LinearRgba::with_luminance a pure scale with no clamp. lighter and darker keep their SDR clamps, and their docs now say so and point HDR users at with_luminance.
• Fixes the From<Laba> for Lcha conversion.
• Updates the channel docs on LinearRgba, Srgba and Xyza. The stated ranges only apply to SDR colors, as discussed in bevy_color: LinearRec2020 #25373.

Behavior notes

SDR colors come out the same, with three exceptions:

• with_luminance on a saturated color can now return components above 1.0 instead of clamping, and the result has the requested luminance exactly.
• A negative component now passes through with_luminance instead of clamping to 0.
• From<Laba> for Lcha drops a stray to_radians() on the Cartesian a and b components. Equal in maths, not bit for bit, so hue can move by a rounding step. It also drops the chroma.clamp(0.0, 1.5) limit.

Migration

See _release-content/migration-guides/bevy_color_hdr_clamps.md.

Testing

• New tests cover the unclamped scaling and the Lcha conversion.
• All bevy_color tests pass.

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This PR was built by me with the assistance of Claude Code w/ Fable 5

[beicause]

The comment is wrong. Even if luminance is within 1.0, components can be greater than 1.0.