feat(local): auto-propose registry patches and track the feedback loop (#117)

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

local/trial-build-patch.R

@@ -0,0 +1,65 @@
+#!/usr/bin/env Rscript
+
+# Acceptance gate for a proposed registry patch (issue #115, step 3): build one
+# package in isolation with the current `local/patches` registry applied, and
+# report whether it succeeds. Nothing is uploaded, archived, or written to the
+# metadata DB -- `patchhash` keeps the trial binary out of the real cache.
+#
+# Must run inside a build-env image (the same image the failing platform uses),
+# because it invokes the real compiler toolchain via bincraft.
+#
+# Usage:
+#   Rscript local/trial-build-patch.R <package> [tag]
+#
+# Exit status: 0 if the patched build succeeds, 1 otherwise -- so it can gate a
+# CI step or a manual pre-merge check.
+
+args <- commandArgs(trailingOnly = TRUE)
+if (length(args) < 1L) {
+  stop("usage: Rscript local/trial-build-patch.R <package> [tag]")
+}
+package <- args[[1L]]
+tag <- if (length(args) >= 2L) args[[2L]] else NULL
+
+script_path <- local({
+  a <- commandArgs(trailingOnly = FALSE)
+  f <- sub("^--file=", "", a[grepl("^--file=", a)])
+  if (length(f) == 1L && nzchar(f)) normalizePath(f) else NA_character_
+})
+patches_dir <- file.path(
+  if (is.na(script_path)) "local" else dirname(script_path),
+  "patches"
+)
+
+suppressPackageStartupMessages(library(bincraft, quietly = TRUE))
+
+cat(sprintf(
+  "Trial build: %s%s with registry %s (no upload/archive/metadata)\n",
+  package,
+  if (is.null(tag)) "" else sprintf(" @ %s", tag),
+  patches_dir
+))
+
+ok <- tryCatch(
+  {
+    bincraft::build_binary_package(
+      package,
+      tag_limit = 1L,
+      patches = patches_dir,
+      archive = FALSE,
+      upload = FALSE,
+      store_build_metadata = FALSE
+    )
+    TRUE
+  },
+  error = function(e) {
+    cat(sprintf("Trial build FAILED: %s\n", conditionMessage(e)))
+    FALSE
+  }
+)
+
+if (ok) {
+  cat(sprintf("Trial build OK: %s builds with the proposed patch.\n", package))
+  q(status = 0)
+}
+q(status = 1)