## Why
Issue #120 (the auto-proposed-patches issue) only lists **auto-proposable** fixes -- currently just the TBB signatures. So a reasonable read of it was "TBB is our only build failure", when in fact three whole categories are simply not shown there:
- **Unclassified failures** -- anything that doesn't match a seeded signature is routed to human triage and never appears (we've only seeded TBB and libuv signatures).
- **Dependency-blocked failures** -- the ~800 RcppParallel dependents (post #121) are still failing; they only show as a log line.
- Human-only signatures (libuv).
These blind spots are exactly where the *next* signatures should come from, so they deserve the same visibility as the proposals.
## What
Extend the feedback-loop tracker to surface the classifier's blind spots:
- **`unclassified_summary()`** -- groups every unknown-signature failure by normalised fingerprint, ranked by build count, capped with an explicit `dropped_groups` count (no silent truncation), each with example packages + platforms. These are the candidates for new `build_signatures()` rules.
- **`blocked_summary()`** -- lists each dependency (e.g. RcppParallel) and how many dependents wait on it.
- `proposal-tracking.R` prints both sections, and a new **`--open-issue`** mode posts/updates a *"Unclassified build failures (needs signatures) (#115)"* Forgejo issue.
- The weekly crow pipeline now runs the tracker with `--open-issue`, so it maintains a second tracking issue alongside the proposals one. Read-only on the DB; the only writes are the two issues.
## Verification
- New tests cover `unclassified_summary` (ranking + both caps) and `blocked_summary`.
- Tracker smoke with a stubbed DB (proposable + blocked + unclassified mix) prints the hit rate, `Blocked on a dependency: RcppParallel: 2 dependent(s)`, and `Unclassified failures ... [2 builds | 2 pkgs] ld: undefined reference ...`.
- Full suite: 95 tests pass; all pre-commit hooks pass (air, prettier, markdownlint, yamllint, validate-patches).
Reviewed-on: #122
Adds the scheduled pipeline for the failure-triage proposer/tracker landed in #117 (issue #115, steps 3 + 4).
## `.crow/weekly-patch-proposals.yaml`
A single (non-matrix) job -- the classifier groups over all of `single_builds`, so unlike the per-platform `weekly-audit-missing` it runs once globally. Weekly it:
1. `propose-patches.R --open-issue` -- posts/updates a Forgejo tracking issue with the auto-proposable registry entries (human reviews, trial-builds, opens the PR; nothing merges).
2. `proposal-tracking.R` -- logs the feedback-loop metrics (signature hit rate, proposed-vs-merged, retirement candidates).
Low blast radius: clones read-only (`REPO_RO_TOKEN`); the only write is the tracking issue via `FORGEJO_TOKEN`. It builds nothing (no B2 keys, no bincraft).
Triggers: `cron: weekly-patch-proposals` (register the cron in the crow UI) or manual with `task=weekly-patch-proposals`, matching the `archive-missed-packages` convention. Documented under "Scheduled run" in `local/patches/README.md`.
`yamllint` and the other pre-commit hooks pass.
Reviewed-on: #118
