feat(local): aggregate blocked-on-dependency reporting by dependency (#131)

## Why

With cascade detection (#128) live, the latest `auto-apply-patches` run did exactly the right thing — **proposed nothing** (`No auto-proposable candidates`) because every failure is a dependency cascade, and it surfaced the ~30 root-cause dependencies to fix.

But the "Blocked on a dependency" list printed **one line per fingerprint group**, so the same dependency repeated (rstan ×4, lpsymphony ×4, salso ×2, BH ×2, GO.db ×2, RcppCWB ×2, …), burying the priority.

## What

Aggregate blocked packages across all groups **by the dependency they wait on**:

- Expose `blocked_map` (package → dependency) from `build_triage_report()`.
- Add `blocked_by_dependency()` — dedupes dependents (a package in two groups counts once) and ranks dependencies by how many distinct dependents they block.
- Proposer and tracker (log + issue) now print one line per dependency, sorted by impact. Replaces the per-group `blocked_summary`.

## Result (same data, aggregated)

```
Blocked on a dependency (3 dependencies block 6 dependents; fix the dependency, not each dependent):
  RcppParallel             3 dependent(s)
  rstan                    2 dependent(s)
  sf                       1 dependent(s)
```

So the real run becomes a crisp, ranked worklist: RcppParallel (894), sf (128), rstan (~96), Rfast (33), clarabel/DescTools (26), Rglpk (22), xgboost (18), …

## Verified

New test covers cross-group aggregation, dedup (a dependent in two groups counted once), the example cap, and ranking. 112 tests pass; hooks pass.

Reviewed-on: #131

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))
   }
 }