Add PDF_PARSER_VERSION constant and propagate parser versioning throughout
the PDF parsing, job, and API layers. Implement normalization of parse state
to ensure compatibility with the current parser version, and enable reuse of
parsed PDF results when possible. Add isPlaybackReady state to document hooks
and TTS player, improving playback UX by disabling controls until content is
ready. Update tests to reflect new playback readiness logic.
BREAKING CHANGE: PDF parse state and job logic now require explicit parserVersion;
older parse states may be treated as pending until reprocessed.
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