Fix reader HTML and native PDF dependency compatibility

[Eoic]

Summary

Fresh reader dependency resolution selects html 0.15.7, which removes the private matches API used by flutter_html 3.0.0 and prevents the reader tests from compiling. The example also locks pdfrx_engine 0.4.5, whose nullable native cache access fails to compile in CI desktop builds.

• Pin html to the compatible 0.15.6 release, with a comment explaining the flutter_html constraint.
• Raise the minimum pdfrx version to 2.4.8, which requires the corrected PDF engine, and refresh the example lockfile to pdfrx_engine 0.4.7.
• Retain the example's Flutter SDK dependency lockfile refresh. Reader source code and public APIs are unchanged.

Validation

• Reproduced the missing matches compilation error locally by resolving html 0.15.7 before applying the constraint.
• Reader: 93 tests passed, 21 skipped; static analysis and formatting passed.
• Example: 2 tests passed; static analysis passed.
• Example: flutter pub get --enforce-lockfile passed without changing the lockfile.
• Example Linux and web release builds passed.
• git diff --check passed.

GitHub CI also builds the example on Linux, Windows, and macOS. The parent submodule update is tracked in PapyrusReader/papyrus#3.

[Copilot]

Copilot wasn't able to review any files in this pull request.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.