v17.12.0

**Changes**

- OCRmyPDF now requires pikepdf 10.2 or later, up from pikepdf 10. This is the
  first release that provides `pikepdf.NamePath`, the `Object.as_int()` family
  of type-safe accessors, and a thread-local `explicit_conversion()`. OCRmyPDF
  uses all three to read optional values out of PDFs it did not write.
- HEIF/HEIC input support is now an opt-in extra, `ocrmypdf[heic]`, instead of
  a default dependency. The `pi-heif` package we previously depended on is
  discontinued; its successor `pillow-heif` bundles libheif, libde265 and x265
  in its wheels, making the wheel as a whole GPLv2-licensed, so we do not
  install it by default. Install `ocrmypdf[heic]` if you feed HEIC images to
  OCRmyPDF. The Docker image and snap do not include it. {issue}`1746`
- A Tagged PDF that is refused because it appears to contain real text now
  exits with code 6 (`already_done_ocr`) instead of code 2 (`input_file`),
  matching the exit code for a PDF with prior OCR. Scripts that check for exit
  code 2 on tagged PDFs need updating. Thanks @BetterAndBetterII.
  {issue}`1551`
- The snap package is now built and published from our CI rather than
  Launchpad, so the `stable` channel tracks releases again instead of being
  frozen at v16.4.2. It is based on core26 and includes a JBIG2 encoder.

**Fixes**

- Text copied or searched in a `--output-type pdfa` file came out as garbage
  (a stable substitution of unrelated characters, or nothing at all) when the
  PDF/A conversion ran through Ghostscript 9.56 through 10.04.x, which
  includes the Ghostscript in Ubuntu 24.04. fpdf2 writes each font's ToUnicode
  CMap as a single `bfchar` block, but the CMap specification allows at most
  100 entries per block, and those Ghostscript releases discard the whole map
  when a block is larger. Any page with more than 100 distinct glyphs was
  affected, which is nearly every page of prose. CFF CID-keyed fonts such as
  Noto Sans CJK have the same problem in their Encoding CMap, where those
  Ghostscript releases then also draw the wrong glyphs. OCRmyPDF now splits
  every CMap block to 100 entries as fpdf2 writes it, for every fpdf2 release.
  fpdf2 2.8.9 will split the ToUnicode blocks itself but not the Encoding
  blocks, so the fix stays in place. Thanks @olafure for the analysis
  ({issue}`1747`, fpdf2 issue py-pdf/fpdf2#1952).
- Words containing `fi`, `ff`, `fl` and similar pairs extracted with the
  letters missing (`con dentiality`, `e ects`) from `--output-type pdfa` files
  when the PDF/A conversion ran through Ghostscript 10.05.0 through 10.06.x.
  Those Ghostscript releases drop ToUnicode entries that expand to more than
  one character (Ghostscript bug 709030, fixed in 10.07.0), and the fpdf2
  renderer's HarfBuzz shaping had been forming optional Latin ligatures whose
  entries do exactly that. Invisible text in scripts that do not need shaping
  is now encoded one glyph per character, so no such entries exist to lose.
  Complex scripts still get shaped, and OCRmyPDF now warns when an affected
  Ghostscript is in use, since their conjunct mappings can still be dropped.
  Thanks @kmn5 ({issue}`1744`).
- A malformed PDF that stores something other than a dictionary at a
  structural key -- `/Resources`, `/Resources /XObject`, `/Root /AcroForm`,
  `/Root /MarkInfo`, `/Root /Names`, `/Root /PieceInfo`, an annotation's `/A`,
  or an image's `/SMask` -- is now tolerated everywhere rather than in the
  handful of places that had been hardened individually. Such a file is read as
  though the key were absent, which is what the well-guarded paths already did.
- An image XObject with no `/Subtype` no longer raises out of the optimizer.
- `PdfInfo` no longer aborts on a `/MarkInfo << /Marked 1 >>`, where a producer
  wrote a flag as an integer instead of a Boolean. Thanks @linhongyu510
  ({issue}`1742`).
- A page `/UserUnit` written as a PDF Real is now read exactly rather than via
  binary floating point, so the digits the file wrote are the digits used.
- `--sidecar` now refuses any spelling of the input or output file, not just a
  byte-identical one. `ocrmypdf in.pdf out.pdf --sidecar ./in.pdf` was accepted
  and then silently overwrote the input PDF with the OCR text. Paths are now
  compared after resolving `.`, `..`, symlinks and filesystem case. Thanks
  @linhongyu510.
- `--deskew` and `--rotate-pages` now warn that they will have no effect when
  combined with `--ocr-engine none`, since skew and orientation are measured by
  the OCR engine. Other image processing options are unaffected. Thanks
  @Anai-Guo. {issue}`1735`
- Installation documentation refreshed for current platforms: Ubuntu 26.04 and
  Fedora 43/44 added, Ubuntu 20.04 dropped, and `uv tool install` shown in
  place of pip.
- Documentation: corrected the list of platforms that ship a JBIG2 encoder,
  which now includes Debian and Ubuntu. Thanks @lcorbasson.
- Release process and CI improvements.