docs(refactor-log): record R6 fix-up under §7

docs/superpowers/refactor-logs/2026-05-06-input-output-unification.md

@@ -128,7 +128,13 @@ navigation: 2 new R26 tests in router.test.ts — BAD_REQUEST on header with ext
 
 ## 7. Open issues / deferred decisions
 
-(populated as encountered)
+- 2026-05-06 / R6 fix-up (commit `9663c82`): Plan 8 left every feature's
+  domain error class with a constructor that did NOT set `this.name`.
+  Plan 9 spec R6 explicitly requires it. Caught by spec reviewer during
+  Task 6 (navigation). Fixed across all 10 error files (auth × 4, blog
+  × 2, marketing-pages × 2, navigation × 2, media × 2). Functionally a
+  no-op (defineErrorMiddleware uses `instanceof`), but ensures correct
+  serialization, stack-trace labels, and JSON inspectability.
 
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