fix(local): route multi-signature packages to human triage in the proposer

A package whose failing builds match more than one auto-proposable signature
(e.g. some logs hit `tbb-stddef-removed` and others `rcppparallel-bundled-tbb`)
produced two candidate registry entries with the same package/platforms/versions
key, so the candidate set failed `validate-patches.R` with "ambiguous duplicate
entries" and the whole proposer run aborted.

Add a pure `dedupe_candidates()` that keeps a package only when it maps to a
single signature and routes genuinely ambiguous packages (conflicting fix tiers)
to human triage instead of guessing between them. The proposer prints the
skipped packages and proceeds with the unambiguous candidates.

local/propose-patches.R

@@ -135,9 +135,25 @@ for (r in report) {
   }
 }
 
+# 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.
+split_candidates <- dedupe_candidates(candidates)
+candidates <- split_candidates$keep
+if (length(split_candidates$ambiguous) > 0L) {
+  cat("\nAmbiguous (multiple signatures) -> human triage, not proposed:\n")
+  for (pkg in names(split_candidates$ambiguous)) {
+    cat(sprintf(
+      "  %s: %s\n",
+      pkg,
+      toString(split_candidates$ambiguous[[pkg]])
+    ))
+  }
+}
+
 if (length(candidates) == 0L) {
   cat(
-    "No auto-proposable candidates (nothing classified, safe, and unregistered).\n"
+    "\nNo auto-proposable candidates (nothing classified, safe, unregistered, and unambiguous).\n"
   )
   q(status = 0)
 }