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/proposal-tracking.R

@@ -127,13 +127,13 @@ if (length(retire) > 0L) {
 # ---------------------------------------------------------------------------
 # Blind spots: failures the classifier could not auto-propose.
 # ---------------------------------------------------------------------------
-blocked <- blocked_summary(report)
+blocked <- blocked_by_dependency(report)
 unmatched <- unclassified_summary(report)
 
 cat("\nBlocked on a dependency (fix the dependency, not each dependent):\n")
 if (length(blocked) > 0L) {
   for (b in blocked) {
-    cat(sprintf("  %s: %d dependent(s) waiting\n", b$blocked_on, b$n_packages))
+    cat(sprintf("  %-20s %5d dependent(s)\n", b$dependency, b$n_packages))
   }
 } else {
   cat("  (none)\n")
@@ -207,8 +207,8 @@ if (do_issue) {
       body_lines <- c(
         body_lines,
         sprintf(
-          "- **%s**: %d dependent(s) waiting (e.g. %s%s)",
-          b$blocked_on,
+          "- **%s**: %d dependent(s) (e.g. %s%s)",
+          b$dependency,
           b$n_packages,
           toString(b$packages),
           if (isTRUE(b$packages_truncated)) ", ..." else ""