7ec2d16f

By: ValentinHirschi <valentin.hirschi@gmail.com>

walk-verify-closure: indexed coverage candidates for wide nodes

A node with at least 256 recorded targets gets an index of its targets by
exact image and by (phase, owner). The coverage check tries the last match,
then the indexed candidates, then the full scan resuming after the last
match. Every candidate is still judged by exact inclusion and the full scan
remains the fallback, so the verdict and the coverage tallies are unchanged;
only the number of inclusion checks drops. The dead prefix holder of the
C-5F rescue drill (1,088,209 successors, fresh and reused targets
interleaved) kept the resumed scan quadratic.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>