Merge origin/main into #560 (fit_preview / camera-look refactors of #528, #583, #568, #616).
`starting_colour` now tries the per-file camera-JPEG fit (with the camera profile when there is
one) first and uses the spectral-sensitivity matrices only when that fit fails, before the neutral
fallback, so no photo with an accepted JPEG fit changes. Tests pin the order through an injectable
fit; docs, parity, ROADMAP and what's new describe it. RENDER_CACHE_VERSION 37 -> 38.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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