feat(local): aggregate blocked-on-dependency reporting by dependency

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`.

local/failing-builds-classify.R

@@ -385,6 +385,7 @@ build_triage_report <- function(
       applies_to = sig$applies_to,
       blocked_on = blocked_on,
       blocked_packages = blocked_packages,
+      blocked_map = if (length(blocked_map) > 0L) blocked_map else NULL,
       fingerprint = names(fp_tab)[[1L]],
       fingerprint_variants = length(fp_tab),
       build_count = nrow(g),