[KnifeLemon]

The other half of #1027 (after #1059). A print user's CMYK Illustrator file → JPG came out RGB: AI goes ai → pdf → jpg through PDFium, which renders RGB only. Same for a CMYK PDF.

What changes

• Detection (Pdf/PdfCmykContent.cs, PdfPig): a document draws in CMYK when its pages (the first 20 selected) use DeviceCMYK colours (k / K, also inside forms), four-channel ICC colour spaces, spot colours (Separation / DeviceN) or CMYK pictures and gradients. Resources are walked with a cap and a cycle check; a PDF PdfPig can't read counts as not CMYK.
• Rendering (GhostscriptConverter.RenderCmykAsync): for a JPG or TIFF target, a CMYK document is drawn by Ghostscript's tiff32nc (8-bit CMYK) at the preset's DPI, cut to the crop box, anti-aliased. DeviceCMYK values come out as the document gives them (checked: 0.1 / 0.2 / 0.3 / 0.4 → 25 / 51 / 76 / 102).
• Profile: with a CMYK output intent (PDF/X), Ghostscript renders into it and the picture carries it; otherwise the picture is untagged CMYK, keeping the numbers for the print shop's own profile (e.g. Japan Color 2001 Coated). Ghostscript's own default profile is removed.
• Encoding: the rendered pages go through the same code as PDFium's (factored out as Encode): quality, resize, _p{n} names, multi-page TIFF, long images, size limits.
• Unchanged: RGB documents, grayscale presets, other targets (PNG, WebP…), and routes.
• No silent colour change: when a CMYK file still comes out RGB because Ghostscript isn't installed or failed on it, the result gets a note saying why (notice.cmyk_needs_ghostscript / notice.cmyk_render_failed, all 19 languages).

Tested

CmykRenderTests (19, run here with the real Ghostscript fetched by build/fetch-engines.ps1 -Only ghostscript, none skipped):

• CMYK PDF → JPG is CMYK, ink values within ±3, no profile, no note; CMYK .ai → JPG is CMYK;
• CMYK PDF → multi-page TIFF with a page range; pages stacked into one JPG on ink-free paper;
• an output intent (SWOP) goes into the JPG byte for byte, values kept;
• RGB PDF → JPG, CMYK PDF → PNG and a grayscale preset stay as before;
• without Ghostscript a CMYK PDF → JPG is RGB with the note;
• detection: CMYK and RGB fills, DeviceCMYK / ICC N=4 / RGB pictures, CMYK vs sRGB output intents, pages outside the selection, broken / missing / truncated PDFs;
• routes: ai → jpg / tiff stay vector, pdfium, pdf → jpg pdfium.

Engines: the whole project 2064 pass, 0 fail (218 skipped for engines not installed here). App localization 214 pass. dotnet format --verify-no-changes clean.

Not in this PR: CMYK EPS / PS → JPG still goes through Ghostscript's pngalpha (RGB).

🤖 Generated with Claude Code