Open GIMP XCF files: layers, groups, masks, modes, channels and selection (#1076)

PhotoCraft reads GIMP's .xcf (and .xcf.gz) files, read only, through a new
reader in photocraft-io written from the public XCF specification: versions
0 to 26, uncompressed, RLE and zlib tiles, 8/16/32-bit integer and
16/32/64-bit float precisions (linear precisions get a linear profile),
pixel layers at any position, groups by item path with pass-through,
layer masks applied or disabled, opacity, visibility, locks, colour tags,
the 2.10 and legacy blend modes mapped to Photoshop's (modes without an
equivalent open as Normal with a note), the ICC profile, resolution,
guides, the comment, extra channels and the saved selection; indexed
images open as RGB. Text, vector and link layers open as their pixels;
layer effects, paths and floating selections are reported.

Fixtures are written by GIMP 3.2 itself (scripts/xcf_fixtures.py), each
with GIMP's merged rendering as a PNG. In gamma-encoded blend and
composite spaces our composite matches GIMP within 0.5/255; GIMP's
linear-light default, its Clip to backdrop and its clamp-after-alpha
for unbounded modes are documented and measured in docs/xcf.md.

README.md

@@ -199,7 +199,7 @@ Every screenshot here is the real app at work on public-domain art, rendered off
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       <h4>🗂️ Formats</h4>
-      PSD and PSB, layered TIFF (Photoshop's layer data in the TIFF, read and written in either byte order), <a href="docs/pdn.md">Paint.NET PDN3 (read, with editable layers and all 14 blend modes)</a>, <a href="docs/ora.md">OpenRaster (read and write, with layers, groups and blend modes, for Krita, MyPaint and GIMP)</a>, <a href="docs/kra.md">Krita .kra (read, with paint layers, groups, blend modes and transparency masks)</a>, SVG (opens as shape layers, places as a vector Smart Object), plus flat PNG, JPEG, TIFF, WebP (lossy and lossless), GIF, BMP, TGA, ICO, QOI, PNM, OpenEXR, Radiance HDR and FITS (astronomy images: 8/16-bit and 32-bit float, gray or RGB, header kept), and the native <code>.pcraft</code> format. Flat formats retain the depths they support: PNG keeps 8/16-bit samples, TIFF keeps 8/16-bit and 32-bit float samples, and OpenEXR keeps half/float samples; JPEG, WebP and the other 8-bit formats reduce higher-depth samples with an export warning. See the <a href="crates/codecs/README.md#capability-matrix-default-build">codec capability matrix</a> for each format. HEIC photos from iPhone and Mac are read-only (included in official builds; optional <code>--features heif</code> for source builds). AVIF is write-only with the optional <code>avif</code> build feature; AVIF decoding and JPEG XL are unavailable. Affinity documents (<code>.af</code>, <code>.afdesign</code>, <code>.afphoto</code>, <code>.afpub</code>) open natively, read only, with artboards, layers, vectors, text, images and masks; effects, adjustments and other unsupported parts are listed in a warning, and Affinity export is unavailable.
+      PSD and PSB, layered TIFF (Photoshop's layer data in the TIFF, read and written in either byte order), <a href="docs/pdn.md">Paint.NET PDN3 (read, with editable layers and all 14 blend modes)</a>, <a href="docs/ora.md">OpenRaster (read and write, with layers, groups and blend modes, for Krita, MyPaint and GIMP)</a>, <a href="docs/xcf.md">GIMP XCF (read, with layers, groups, masks, blend modes, channels and the selection)</a>, <a href="docs/kra.md">Krita .kra (read, with paint layers, groups, blend modes and transparency masks)</a>, SVG (opens as shape layers, places as a vector Smart Object), plus flat PNG, JPEG, TIFF, WebP (lossy and lossless), GIF, BMP, TGA, ICO, QOI, PNM, OpenEXR, Radiance HDR and FITS (astronomy images: 8/16-bit and 32-bit float, gray or RGB, header kept), and the native <code>.pcraft</code> format. Flat formats retain the depths they support: PNG keeps 8/16-bit samples, TIFF keeps 8/16-bit and 32-bit float samples, and OpenEXR keeps half/float samples; JPEG, WebP and the other 8-bit formats reduce higher-depth samples with an export warning. See the <a href="crates/codecs/README.md#capability-matrix-default-build">codec capability matrix</a> for each format. HEIC photos from iPhone and Mac are read-only (included in official builds; optional <code>--features heif</code> for source builds). AVIF is write-only with the optional <code>avif</code> build feature; AVIF decoding and JPEG XL are unavailable. Affinity documents (<code>.af</code>, <code>.afdesign</code>, <code>.afphoto</code>, <code>.afpub</code>) open natively, read only, with artboards, layers, vectors, text, images and masks; effects, adjustments and other unsupported parts are listed in a warning, and Affinity export is unavailable.
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       <h4>🪄 The everyday essentials</h4>