Show per-filter preview swatches in PhotosCrop Filters strip

[muukii]

What

Each filter in the PhotosCrop Filters strip now renders a live thumbnail of the photo with that filter applied (Photos/Instagram-style), replacing the text-only pills — including an "Original" swatch.

How

• New PhotosCropFilterThumbnail.swift — a @concurrent single-swatch renderer on a dedicated wide-gamut CIContext whose working/output color spaces match the editing canvas (extendedLinearDisplayP3 → displayP3), so a swatch and the live canvas resolve the same colors. It center-square-crops the session thumbnail, never upscales past the source, and threads a source-relative radiusReferenceExtent so host-supplied blur/sharpen presets preview at the correct strength.
• Per-cell lazy rendering — each PhotosCropFilterChip renders its own swatch in a .task into its @State. A LazyHStack defers off-screen first renders and retains created chips, so scrolling never re-renders. No shared cache or stringly-typed dictionary.
• Selected chip auto-centers via .scrollPosition(id:anchor:) on selection change (clamps at the ends, so Original / the last filter rest at the edge).
• Selection ring uses a dark shadow halo so it stays legible on bright swatches (Original / Mono / Noir on light photos).
• Swatches preview the filter only — not the user's manual Adjust edits, blur mask, or current crop — matching the Photos/Instagram strip.

Notes

• Uses the iOS 17 scrollPosition(id:anchor:) modifier (not deprecated). The iOS 18 ScrollPosition struct would need if #available since the deployment floor is iOS 17.

Verification

• BrightroomUI builds clean in Swift 6 language mode.
• Verified on the iOS Simulator: all swatches render distinctly (Mono/Noir desaturated, Warm/Cool shifted, Fade washed, Vivid/Dramatic as expected), off-screen chips render lazily when scrolled into view, the selection ring is visible on bright swatches, and selecting a chip scrolls it to center.

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