With cascade detection live, a run surfaced ~30 root-cause dependencies, but the
"blocked on a dependency" list repeated the same dependency once per fingerprint
group (rstan x4, lpsymphony x4, salso x2, ...), burying the priority.
Aggregate across all groups by the dependency each dependent waits on: expose
`blocked_map` (package -> dependency) from build_triage_report, and add
`blocked_by_dependency()` which dedupes dependents and ranks dependencies by how
many they block. One line per dependency, sorted by impact, in the proposer and
tracker (log + issue). Replaces the per-group `blocked_summary`.
pat-s
1b57d45c80