refactor(api)!: one way to open a file

[andiwand]

🤖 Generated with Claude Code

PR 6 of the v7 API plan.
Stacked on #835 — dropping DocumentFile::type/::meta there is what
leaves these constructors with no callers. Review #835 first; base retargets to
main when it merges.

odr::open had a twin

odr.cpp:203-230 was six one-line return DecodedFile(...). Not a wrapper with
a purpose — the same call under a second name, and the split had already spread:

• JNI exposed both (Odr.open(path) and new DecodedFile(path)), with
  the javadoc admitting it: "Obtain via Odr#open or the constructors"
• Apple went through the constructors
• Python bound five DecodedFile.__init__ overloads and six open functions

The bodies move into odr::open. DecodedFile keeps only the constructor that
wraps an already-decoded impl.

DocumentFile was never a second kind of open

DocumentFile::from_disk, ::from_memory, DocumentFile(File) and
DocumentFile(path) all reached open_strategy::open_document_file — which
repeats open_file's cascade engine for engine for zip and cfb, differing
only in the fallback, and both ends throw NoDocumentFile. as_document_file()
throws it too (file.cpp:278).

So open(f).as_document_file() is not an approximation of DocumentFile(f) —
same engines, same order, same exception. 73 lines of open_strategy.cpp go
with it (737 → 664).

Bindings

	gone	replacement
Java	new DecodedFile(path), (path, as), (file), new DocumentFile(path)	Odr.open(...), .asDocumentFile()
Python	DecodedFile(...) ×5, DocumentFile(...) ×2, DocumentFile.from_disk / .from_memory	pyodr.open(...), .as_document_file()
ObjC	surface unchanged; decode(...) now calls odr::open	—

Java gains Odr.open(File), which is exactly what the removed
DecodedFile(File) constructor was — otherwise an opened File would have had
no public way to decode.

About the test churn

~85 call sites moved. I rewrote them with a balanced-paren pass rather than a
regex, because a first attempt with a naive regex silently mangled a function
declaration (DecodedFile csv_file(const std::uint32_t rows, ...) became
DecodedFile csv_file = open(const std::uint32_t rows, ...)). The final pass
guards against parameter-declaration shapes and leaves impl-handle
constructions alone — DecodedFile(xml_file(content)) and the rtf/svg cases
pass a shared_ptr to an internal impl, so they keep the impl constructor.

Two internal tests called open_strategy::open_document_file directly and now
go through open_file + as_document_file().

Verified

Full build clean. Full gtest suite: 1456 passed, 6 skipped (the same six
that skip on main). 69 python tests and the JNI junit suite pass.

Migration

DecodedFile(path)                →  odr::open(path)
DecodedFile(file, as)            →  odr::open(file, as)
DocumentFile(path)               →  odr::open(path).as_document_file()
DocumentFile::from_memory(bytes) →  odr::open(File::from_memory(bytes)).as_document_file()

new DecodedFile(path)   →  Odr.open(path)
new DecodedFile(file)   →  Odr.open(file)
new DocumentFile(path)  →  Odr.open(path).asDocumentFile()