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/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),