feat(api): open a file from memory, and take a File wherever a path is taken

[andiwand]

🤖 Generated with Claude Code

Split out of #661 — no wasm in here.

Every input entry point took a std::string path, so a caller holding bytes and
no path — a download, a browser upload, a decrypted payload — had no way in.
internal::MemoryFile already did the right thing, but no binding could reach
it.

• File::from_disk(path) and File::from_memory(data), and the same pair on
  DocumentFile so the common case — bytes straight to a document — needs no
  intermediate handle. Named factories rather than constructors, because
  File(const std::string &) already means path.
• A File overload wherever a path is taken: DecodedFile::list_file_types /
  mimetype, DecodedFile(File, DecodePreference), DocumentFile::type /
  meta and its constructor, and the odr:: free functions. The path forms
  become one-line forwards to from_disk.
• magic::mimetype gains a File overload — it was the last place that reached
  for DiskFile on its own. The internals keep taking
  const std::shared_ptr<abstract::File> &: open_strategy hands the file on to
  ZipFile / TextFile / PdfFile and friends, which store it, so that is the
  parameter type that says what actually happens.
• The python binding exposes all of it. The File forms of open carry
  py::call_guard<py::gil_scoped_release> like their path siblings — decoding is
  long-running either way — and DocumentFile.from_memory copies the bytes out
  under the GIL before releasing it for the decode.

Follow-up, not in scope: the path overloads are now pure forwards, so they can
be deprecated and removed once cli/, python/, jni/, apple/ and test/
are migrated off them. CI builds with -Werror, so that order is forced.
File::from_disk stays.

Tested: full C++ suite (764 passed, 9 skipped at the top of the stack) and
the pytest suite (59 passed), which gains coverage for both factories, the
File entry points, binary round-tripping through from_memory, and that the
File and path forms agree.

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