Enhance Linear REC.2100 support: update HDR handling and add tests

[Aditya30ag]

Summary

rec2100-linear inherited the default SDR [0, 1] range from RGBColorSpace. HDR values above 1 were reported as out of gamut, and gamut mapping turned them into white.

const c = new Color("rec2100-linear", [0.5039, 2.389, 0.0447]);
c.inGamut();         // false (expected: true)
c.toGamut().coords;  // [1, 1, 1] (expected: unchanged)

Change

Following the approach suggested in the issue, rec2100-linear now defines refRange: [0, 1] instead of range:

• The space is unbounded, so no gamut mapping happens to HDR values.
• Percentages still resolve against 0 to 1 as before.
• In src/toGamut.js, the ray-trace lower bound for PQ and HLG read .range[0] from the linear space. It now falls back to refRange, so they keep working.

Testing

• Added tests to test/in_gamut.js and test/gamut.js. Without the source change they fail.
• Full suite passes.
• I compared gamut-mapping output before and after for rec2100-pq, rec2100-hlg, rec2020, and p3 using the css, raytrace and clip methods. Only rec2100-linear changed.

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[Aditya30ag]

Fixes #630. Related: color-js/apps#11