Fix #320: CMYK images stay CMYK from PDF import to PDF export

[bflatastic]

Fixes #320. CMYK images in a PDF now keep their CMYK samples when the file is opened, and PDF export writes them as CMYK. They used to become RGB with no warning.

• Model (vectorcraft-doc::cmyk): a CMYK image is stored as a four-component JPEG (Adobe's inverted samples) or as a Deflate CMYK TIFF. ImageBlob::is_cmyk / ::cmyk read the ink amounts, capped at 64 megapixels, and ::cmyk_tiff writes the TIFF.
• PDF import: DeviceCMYK, CalCMYK and four-component ICC images keep their samples. A JPEG with Decode [1 0 1 0 1 0 1 0] is kept byte for byte. Any other is decoded (8 or 16 bits, Decode array applied) and stored as a CMYK TIFF. Masked CMYK images and 1/2/4-bit ones still open in RGB, now with a warning. Placing a CMYK TIFF also keeps it CMYK.
• PDF export: CMYK images are written as DeviceCMYK, or as ICC-based with the CMYK profile when colours are tagged (PDF/X-3, PDF/X-4, profiles: all). A CMYK JPEG that is not resampled or recompressed passes through unchanged. Other CMYK images are resampled in CMYK and compressed as ZIP or as a CMYK JPEG (jpeg-encoder). The CMYK JPEG encoder also replaces the old ZIP fallback and its warning for images converted to CMYK. Output conversion treats CMYK images like CMYK colours:
  • Preserve Numbers and PDF/X-1a keep their numbers.
  • Convert to Destination converts them to another CMYK profile or to RGB, through a 17⁴ cms::CmykLut. The renderer's ink table is now built on the same LUT.
  • A CMYK image that can't be decoded is reported in warnings.
• Canvas: CMYK images are shown through the working CMYK profile, the same way CMYK fills are, where before they used a naive conversion. In ink planes they paint their own inks (CMYK raster exports, CMYK blending).
• SVG export: TIFF images are written as PNG, so browsers can show them.
• Illustrator parity: Output › No Conversion "preserves color data as is" (https://helpx.adobe.com/illustrator/using/pdf-options.html).
• docs/mcp.md and ROADMAP.md (Shipped so far › Files) are updated.

Checks

• Rebased on origin/main @ ce12b4b.
• cargo fmt --all --check, cargo clippy -p vectorcraft-{pdf,render,doc,color,svg,engine} --all-targets -D warnings: clean.
• cargo test for doc, color, render, pdf and svg: 781 passed. vectorcraft-engine: 1356 passed.
• New tests:
  • pdf/tests_cmykimages.rs (4): JPEG pass-through for default, Preserve Numbers, X-1a, X-3, X-4 and profiles: all. Flate (8/16-bit, inverted Decode), non-Adobe DCT and ICC N=4 images. Mask and 4-bit warnings. Conversions (other CMYK profile, RGB), CMYK JPEG recompression and resampling.
  • doc/cmyk.rs (3), render/ink.rs (1: the image matches a CMYK fill on screen and paints its exact inks), engine fileio (1: a placed CMYK TIFF stays CMYK), svg/tests/options.rs (1).
  • The tests fail without the fix: with the import change disabled, all 4 PDF tests fail; with the render change disabled, the canvas test fails ([230,138,0] vs [229,151,16]).
• cargo xtask layers, cargo xtask brands, cargo xtask wasm (19 crates ok) pass. cargo check --target x86_64-unknown-linux-gnu and aarch64-apple-darwin pass on doc/pdf/render/svg.
• Live run with the issue's case, using a debug vectorcraft-cli on a generated PDF (PIL CMYK JPEG with Decode [1 0 …] plus a Flate CMYK image plus a CMYK fill):
  • info reports colorMode: cmyk.
  • convert gives DeviceCMYK for both images, with the source JPEG bytes embedded unchanged.
  • exportPdf pdfX3 and pdfX4 give [/ICCBased …] with N=4. pdfX1a gives DeviceCMYK.
  • All four runs return warnings: [].
  • The PNG render shows the image at (255,217,155), which matches the reporter's poppler reading of the source file.

🤖 Generated with Claude Code

[bflatastic]

Full cargo xtask ci passed on the integrated stack; merging.