feat(file): hand out the thumbnail the package already carries

[andiwand]

🤖 Generated with Claude Code

Closes #21 — which asked whether we would have to render documents ourselves, and the answer in the comment thread was "there is a thumbnail.png inside which would be easy to use". It is: the archive we already open has it.

/// @brief The preview image the file carries, if it carries one.
[[nodiscard]] std::optional<File> DocumentFile::thumbnail() const;

• ODF names the part by convention: Thumbnails/thumbnail.png.
• OOXML names it by package relationship — .jpeg, .png, .emf and .wmf all occur — so the relationship is read rather than a filename guessed.

Nothing is rendered to produce it, so it is as current as the last save by an application that writes one, and a great many files carry none — which is a nullopt, not an error. An encrypted package carries none until it is decrypted.

Along the way

parse_relationship_target did not handle the package root: / is not a part, so AbsPath("/").parent() throws "absolute path violation". /_rels/.rels is where the package's own relationships live and its targets resolve against the root, so the two helpers now say so.

Bindings

Mirrored in JNI (DocumentFile.thumbnail(), null for absent), Apple (DocumentFile.thumbnail, a nullable ODRFile * following the ODRImage.file pattern) and Python (DocumentFile.thumbnail(), None for absent). Not in wasm, whose file surface is session-based and exposes no odr::File at all — a separate question.

Verification

• core: ./odr_test --gtest_filter='-all_test_files/*' — 1117 tests, all pass, including three new ones. The OOXML one builds a package inline with the thumbnail at the deliberately unconventional docProps/preview.emf, so it proves the relationship is what is read.
• JNI: ctest --test-dir build/jni — 44 tests, thumbnail() among them.
• Python: ctest --test-dir build/python — test_document_file_thumbnail passes; it asserts both the absent case (the minimal odt the fixture builds) and the present one.
• Apple: macOS slices built, ODR_XCFRAMEWORK=OdrCoreObjC.xcframework swift test — 22 tests, ThumbnailTests among them.

The shared mixed-layout.odt fixture is real LibreOffice output and carries a 428-byte Thumbnails/thumbnail.png, so all three binding suites test against it without a new fixture.