fix(local): make the trial-build gate detect non-throwing build failures (#130)

## Critical: the gate was false-green

The latest run printed **`3/3 passed on alpine-324`** while all three builds actually **failed** (their `rstan` dependency won't compile). A false-green gate would let broken registry entries merge — worse than no gate.

Root cause: `bincraft::build_binary_package()` catches build failures internally and **returns `"error"`** for the failed tag rather than throwing. The gate's `tryCatch` only treated a *thrown* exception as failure, so every non-throwing failure looked like a pass.

Fix: inspect the return value. A tag passes only if the flattened result is non-empty and contains no `"error"` sentinel; a thrown error still counts as failure. Verified the verdict against `error`/`skipped`/`TRUE`/`list(success=TRUE)`/`NULL`/mixed inputs.

With this, the current rstan-blocked entries will correctly show **0/3 (red)** — which is the right answer until rstan builds.

Reviewed-on: #130

local/trial-build-registry.R

@@ -117,23 +117,34 @@ results <- vapply(
   pkgs,
   function(pkg) {
     cat(sprintf("\n=== trial build: %s ===\n", pkg))
-    tryCatch(
-      {
-        bincraft::build_binary_package(
-          pkg,
-          tag_limit = 1L,
-          patches = patches_dir,
-          archive = FALSE,
-          upload = FALSE,
-          store_build_metadata = FALSE
-        )
-        TRUE
-      },
+    # bincraft::build_binary_package() catches build failures internally and
+    # RETURNS "error" for the failed tag rather than throwing, so a green gate
+    # must inspect the return value -- checking only for a thrown exception
+    # reports a broken build as passing.
+    res <- tryCatch(
+      bincraft::build_binary_package(
+        pkg,
+        tag_limit = 1L,
+        patches = patches_dir,
+        archive = FALSE,
+        upload = FALSE,
+        store_build_metadata = FALSE
+      ),
       error = function(e) {
-        cat(sprintf("FAILED %s: %s\n", pkg, conditionMessage(e)))
-        FALSE
+        cat(sprintf("FAILED %s (threw): %s\n", pkg, conditionMessage(e)))
+        "error"
       }
     )
+    flat <- as.character(unlist(res))
+    ok <- length(flat) > 0L && !("error" %in% flat)
+    if (!ok) {
+      cat(sprintf(
+        "FAILED %s: build did not succeed (result: %s)\n",
+        pkg,
+        if (length(flat) > 0L) toString(flat) else "<empty>"
+      ))
+    }
+    ok
   },
   logical(1L)
 )