Fix dependency-handling bugs: RAR probe, PDF guard, config errors

[ajslater]

Seven bugs in how comicbox detects and reports its external dependencies, found while planning a comicbox --doctor command.

Changes

• RAR support is probed, not guessed. is_unrar_supported() / check_unrar_executable() now extract the compressed member of a bundled 110-byte RAR5 archive (comicbox/_rar_probe.rar) through whichever tool rarfile selects, via the new cached comicbox._rar.rar_unsupported_reason(). Checking which("unrar") was wrong in both directions, and so is trusting rarfile.tool_setup(). rarfile 4.5's bsdtar command line opens a file named --, and Homebrew's 7zz has no RAR codec. Both pass rarfile's tool check but extract nothing, so on every Mac the check would report CBR support that doesn't work.
• Tool failures map to UnsupportedArchiveTypeError. RarCannotExec, and tool failures while constructing a RarFile (RAR3 compressed comments), now map the way BadRarFile already did, when the probe says the host can't extract.
• is_pdf_supported() returns PDF_ENABLED. It used to check "pdffile" in sys.modules, which is also true when the guard rejected pdffile or the embedding app imported it.
• A broken PDF extra no longer breaks the rest of comicbox. _pdf.py survives any pdffile import failure; pymupdf asserts its libmupdf version at import, which raised past the except ImportError. It warns unless pdffile is simply not installed.
• A malformed user config.yaml no longer drops the package defaults. Previously validation then died on comicbox.paths not found.
• Config errors are typed and print cleanly. They raise a new ConfigurationError(ComicboxError, ValueError), so callers catching ValueError are unaffected. The CLI prints those and confuse's ConfigError as one yellow line instead of a traceback. Messages are escaped so rich no longer eats [digital] as markup.
• online.tuning.per_source blocks are type-checked. They use the existing but previously unused tuning template. Null blocks are allowed, and unknown source names warn and are skipped.
• _archive_errors() tolerates an unimportable rarfile or py7zr. It previously raised inside the except clause and aborted batch reads.
• Stale zipremove comment corrected. Python 3.14 has no ZipFile.remove / repack.

Codex impact (after it bumps comicbox)

• Codex calls Comicbox.is_unrar_supported() (codex/librarian/fs/filters.py), which now runs one cached probe subprocess per process.
• A host whose only RAR tool can't extract stays CBR-unsupported.
• A host with a working tool other than unrar becomes supported.

Testing

• make fix, make lint (ruff, basedpyright 0 warnings, vulture, complexipy) and make ty are clean.
• radon cc --min C and radon mi --min B on the changed files print nothing.
• make test: 2289 passed, 1 pre-existing skip.
• New tests: tests/unit/test_rar_tool_probe.py, test_pdf_import_guard.py, test_config_errors.py, test_process_archive_errors.py, plus three CLI cases in tests/cli/test_cli_exit_code.py.
• The process-error, PDF-guard and CLI tests fail against unfixed develop, run in a throwaway worktree. The RAR-probe and config tests import new names, so that check couldn't apply to them.
• A built wheel contains comicbox/_rar_probe.rar.

🤖 Generated with Claude Code