Add lossless JPEG compression for raw denoise DNG output

[andriiryzhkov]

Sibling to #21435 (TIFF compression for denoise/upscale). That PR handles the TIFF outputs; this one does the equivalent for raw denoise, which writes DNG.

The naïve fix – COMPRESSION_JPEG + tile writes – doesn't work: libtiff's JPEG codec hard-rejects 16-bit. So I encode each 256×256 tile with libjpeg-turbo (which does support 16-bit lossless), tell libtiff COMPRESSION_ADOBE_DEFLATE so it accepts the setup, hand it the JPEG bytes via TIFFWriteRawTile() (libtiff stores opaque bytes without invoking its codec), then post-patch the Compression tag from 8 → 7 in the closed file. The on-disk data is genuine lossless JPEG; the deflate codec is never actually invoked. Documented in detail at the top of src/imageio/imageio_dng.c.

A new compression combo in the raw-denoise output panel (uncompressed / lossless JPEG, default lossless) mirrors the TIFF combo from #21435.

File size on a noisy ISO 6400 Canon R7: ~50 MB → ~41 MB. Cleaner sources compress better.

[andriiryzhkov]

Moving to draft — the implementation hits a Linux distribution problem I didn't catch before.

jpeg_enable_lossless() requires libjpeg-turbo 3.0+ (Oct 2023). The AppImage builds on Ubuntu LTS, which ships libjpeg-turbo 2.x. So as-is, every Linux user via AppImage would get a build failure or "no compression" fallback.

Path options:

1. Feature-detect + fallback to uncompressed — Linux AppImage users get no compression. Defeats the point.
2. Switch to DEFLATE compression — rejected: rawspeed enforces the DNG spec strictly and refuses to read integer DNG with DEFLATE (DngDecoder.cpp:362), so the files wouldn't open in darktable itself.
3. Vendor liblj92 — small standalone lossless JPEG encoder (used by Magic Lantern). True lossless JPEG everywhere, spec-compliant, ~1500 lines in src/external/. Even though this looks like the best option, it's a lot of effort compared to the size of the issue it solves.

Parking the PR until I have a clearer answer.