#644 Enumerate BOSS sink dummy edges with rank/select

Enumerate BOSS sink dummy edges with rank/select instead of scanning every edge, preserving the count-only path and existing mask bits. Assert the two leading sentinels and start one-based select at occurrence 3.

Include regression coverage for all four BOSS states, empty graphs, small paths and pre-populated masks, plus a benchmark of the production method including mask allocation. This is an AI-assisted extraction of Gunnar's sink-marking change.