## 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
Implements steps 3 + 4 of #115, building on the classifier merged in #116. Now that bincraft **v4.4.3** applies registry `patch`/`makevars`/`configure_args` to the *target* package build (previously deps-only), a trial patched build is a meaningful acceptance gate, so the "propose" half is viable.
## Step 3 — propose, do not apply
- **`local/propose-patches.R`** — for each classified, safe fix affecting a package with no current registry entry, emits a pre-filled `registry.json` entry and validates the candidate set against a *temporary* merged registry (the real one is never touched unless asked).
- default: print candidates + validation, **take no action**
- `--write`: append entries to `registry.json` + the proposals ledger (you commit + open the PR)
- `--open-issue`: post/update a Forgejo tracking issue (reuses the weekly-audit `httr2` + `FORGEJO_TOKEN` pattern)
- **`local/trial-build-patch.R`** — isolated bincraft build of one package with the registry applied (no upload/archive/metadata; `patchhash` keeps it out of the real cache). Exit 0/1, so it gates a CI step or manual pre-merge check.
The human gate stays: nothing merges. Acceptance = `validate-patches.R` passes (checked automatically) **and** the trial build succeeds. Novel source diffs and unknown signatures are never proposed (they carry `auto = FALSE`).
## Step 4 — feedback loop
- **`local/proposal-tracking.R`** (read-only) — signature hit rate (builds/pkgs/addressed/open per signature), proposed-vs-merged (a proposal counts merged once its package is in the registry), and retirement candidates (registry entries whose package no longer fails, i.e. likely fixed upstream).
- **`local/proposal-tracking-lib.R`** — the pure metric/ledger helpers.
## Supporting changes
- Refactored the classify helpers to expose a pure `build_triage_report()` + a list-returning entry builder; `failing-builds-report.R` now renders from the shared function (no behaviour change).
- `validate-patches.R` gains optional `PATCH_DIR`/`REGISTRY_FILE` overrides (backward-compatible) so a candidate registry can be validated in isolation.
- Documented the propose/trial-build/tracking workflow in `local/patches/README.md`.
## Verification
- 71 unit tests pass (incl. new `test-proposal-tracking-lib.R`) under the Dockerized R 4.5.3 build env.
- All pre-commit hooks pass (`air-format`, `validate-patches`, prettier, etc.).
- Smoke-tested all three entrypoints end-to-end with a stubbed DB: dry-run, `--write` (produces a registry that passes the canonical validator + a valid ledger, then reverted), and the tracker.
Closes#115
Reviewed-on: #117
