feat: add font conversion dependencies and test fixtures

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/fixtures/FONT_ATTRIBUTION.md

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+# Font fixture attribution
+
+`sample.otf` and `sample.ttf` are the Regular weight of
+[Source Code Pro](https://github.com/adobe-fonts/source-code-pro) by Adobe,
+licensed under the SIL Open Font License 1.1 (see that repo's
+`LICENSE.md`). Both are the same font family deliberately, so tests can
+assert glyph-count fidelity (1568 glyphs) across otf/ttf/woff conversions
+of "the same font" rather than comparing unrelated files.
+
+`sample.dfont` is `Tests/ttx/data/TestDFONT.dfont` from
+[fonttools/fonttools](https://github.com/fonttools/fonttools), MIT
+licensed. It is a small (3.5KB) fixture containing one real embedded
+`sfnt` TrueType resource (7 glyphs) — used to exercise this project's
+dfont-container extraction code without needing a large real-world
+font suitcase.