feat(traversal): add multi-frontier adaptive salience feedback expansion

Three-phase algorithm for salience-driven path discovery:
- Phase 1: path potential discovery until M paths found
- Phase 2: salience feedback expansion with adaptive priority
- Phase 3: terminate when salience plateau + diversity threshold met

Priority adapts based on accumulated salience feedback from discovered
paths. Includes unit tests for phase transitions and path discovery.

Author: Mearman