fix(local): stop proposing per-dependent patches for dependency-cascade failures

The `--open-issue` run proposed 800+ entries, all `rcppparallel-bundled-tbb` on
ubuntu-2604, each pre-filling `RcppParallel/disable-tbb.patch` for an unrelated
package (ACEsimFit, AovBay, ...). Those packages fail only because their
RcppParallel *dependency* does not build there, so bincraft records
RcppParallel's TBB error text against each dependent and they all match the
signature. Applying a RcppParallel source patch to a dependent's source is not
just noise -- it is broken (the diff targets RcppParallel's Makevars.in).

Add an optional `applies_to` field pinning a package-specific fix to the package
it targets. A signature whose fix is a curated per-package source patch
(rcppparallel-bundled-tbb -> RcppParallel) is only ever proposed for that
package; other matching packages are downstream failures reported as "blocked on
<dependency>" and never proposed a bogus entry. Fix the dependency once and the
whole cascade clears.

- gate proposal generation in build_triage_report on applies_to, add blocked_on
- surface blocked groups in the report and proposer instead of a misleading
  "already registered" note
- cover both the dependent-blocked and RcppParallel-itself-proposed cases

local/propose-patches.R

@@ -135,6 +135,21 @@ 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) !is.null(r$blocked_on), report)
+if (length(blocked) > 0L) {
+  cat("\nBlocked on a dependency (fix the dependency, not each dependent):\n")
+  for (r in blocked) {
+    cat(sprintf(
+      "  %s: %d package(s) fail because %s does not build\n",
+      toString(r$blocked_on),
+      length(r$packages),
+      toString(r$blocked_on)
+    ))
+  }
+}
+
 # A package that maps to more than one auto-proposable signature is ambiguous
 # (conflicting fix tiers) and would collide on the same registry key; route it
 # to human triage instead of emitting both.