stage3: unify to cleanup; MIR return-defer; docs+smokes updated; LLVM(harness): finalize_phis ownership, ret.py simplified, uses-predeclare; cleanup return override green; method-postfix cleanup return WIP (PHI head)

docs/quick-reference/syntax-cheatsheet.md

@@ -213,7 +213,7 @@ try {
     local result = riskyOperation()
 } catch (error) {
     print("Error: " + error.message)
-} finally {
+} cleanup {
     cleanup()
 }
 ```