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/propose-patches.R

@@ -148,21 +148,20 @@ for (r in report) {
   }
 }
 
-# Groups blocked on a dependency (e.g. RcppParallel dependents) are reported,
-# not proposed: fixing the named dependency clears them all at once.
-blocked <- Filter(
-  function(r) length(r$blocked_packages) > 0L,
-  report
-)
+# Packages blocked on a dependency are reported (aggregated by dependency,
+# ranked by impact), not proposed: fixing the named dependency clears the batch.
+blocked <- blocked_by_dependency(report)
 if (length(blocked) > 0L) {
-  cat("\nBlocked on a dependency (fix the dependency, not each dependent):\n")
-  for (r in blocked) {
-    cat(sprintf(
-      "  %s: %d package(s) fail because %s does not build\n",
-      toString(r$blocked_on),
-      length(r$blocked_packages),
-      toString(r$blocked_on)
-    ))
+  n_blocked_pkgs <- length(unique(unlist(
+    lapply(report, function(r) r$blocked_packages)
+  )))
+  cat(sprintf(
+    "\nBlocked on a dependency (%d dependencies block %d dependents; fix the dependency, not each dependent):\n",
+    length(blocked),
+    n_blocked_pkgs
+  ))
+  for (b in blocked) {
+    cat(sprintf("  %-20s %5d dependent(s)\n", b$dependency, b$n_packages))
   }
 }