Make the Periphery scan blocking (#1648)

The job shipped with its own promotion condition — "drop
continue-on-error once the baseline proves stable" — and the baseline
has now been stable across a month of merges since it landed July 8
(#1410). Known macOS-scan false positives stay suppressed by the
committed baseline; --strict already fails the step on NEW dead code,
so the only change is that the failure now blocks instead of annotating.

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/periphery.yml

@@ -11,9 +11,11 @@ jobs:
     name: Periphery scan
     runs-on: macos-latest
     timeout-minutes: 30
-    # Advisory, like SwiftLint (#1361): findings annotate the PR but don't
-    # block merges. Drop continue-on-error once the baseline proves stable.
-    continue-on-error: true
+    # Blocking, like SwiftLint since #1646. The baseline (committed July 8)
+    # has been stable across a month of merges — the "drop continue-on-error
+    # once the baseline proves stable" condition this job shipped with is met.
+    # Known macOS-scan false positives stay suppressed by the baseline; only
+    # NEW dead code fails the job (--strict below).
 
     steps:
       - name: Checkout code