feat(pdftract-2iyk): implement classifier engine
Implements Phase 5.6.2 classifier engine that evaluates document type
profiles against extracted feature signals.
- ClassifierEngine: evaluates profiles, computes normalized scores,
returns highest-scoring profile above threshold
- FeatureSignals: struct containing all metrics for predicate matching
- ClassificationResult: document_type, confidence, reasons, runner_up
- Score normalization: matched_weight / total_weight to [0, 1]
- Predicate evaluation: all MatchPredicate variants supported
- Regex caching: OnceLock-based cache for TextMatchesRegex
- Unit tests: 28 tests covering invoice, scientific_paper, unknown
classification, score normalization, tie-breaking, determinism
Closes: pdftract-2iyk
865429d5f6