Add PageClassification struct wrapping PageClass with confidence
and optional hybrid_cells metadata for Phase 5.1 classifier.
- struct: PageClass + f32 confidence + Option<BTreeSet<(u8, u8)>>
- constructor with debug_assert on confidence range (INV-8)
- serde derives with skip_serializing_if for hybrid_cells
- comprehensive unit tests for all acceptance criteria
Closes: pdftract-390fn
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
