test(ir): pin both lambda arms of the error-flow pass (A5.2 scaffolding review fix)

Codex review of 95895a3 found 1051 reached neither lambda arm it claimed to
pin: the lambda arrived only as a var_decl initializer, which routes through
checkCleanupNode's `.var_decl` arm -> cleanupReject(lambda) -> early-return
(a lambda literal is not failable), so the `.lambda` stop never ran; and its
accepted-direction `if !err` guard would still pass with flowExpr's lambda
recursion removed.

Scaffolding-only fix (no compiler change):
- 1051: add a bare lambda STATEMENT `() -> !E { failing(); };` in the cleanup
  body so checkCleanupNode sees a `.lambda` node directly and stops (the bare
  failable inside is accepted; were the arm to recurse it would reject like
  1052). Output byte-identical — only the .sx gained the statement.
- 1053-errors-nested-lambda-liveness-reject (exit 1): an E1.8 value-slot read
  inside a never-called nested lambda, rejected only because flowExpr recurses
  via `.lambda => analyzeFnBody`. Remove that arm and the diagnostic vanishes
  -> suite fails. This is the discriminating negative 1051 lacked.

Gate: zig build test, bash tests/run_examples.sh -> 361/0.

examples/1053-errors-nested-lambda-liveness-reject.sx

@@ -0,0 +1,31 @@
+// Value-slot liveness (ERR step E1.8) is analysed inside a nested lambda as its
+// OWN boundary: `flowExpr` recurses into a lambda literal via `analyzeFnBody`.
+// Reading a failable's value slot inside the lambda where its error is NOT
+// proven absent is rejected — even though the lambda is never called and the
+// outer function proves nothing for it.
+//
+// Negative counterpart to 1051(b): were `flowExpr`'s `.lambda` recursion
+// removed, the lambda body would go un-analysed and this read would slip
+// through. The program never runs (exit 1).
+
+#import "modules/std.sx";
+
+E :: error { Bad }
+
+parse :: (n: s32) -> (s32, !E) {
+    if n < 0 { raise error.Bad; }
+    return n * 10;
+}
+
+build :: () {
+    emit := () -> s32 {
+        v, err := parse(5);
+        return v;               // REJECTED: err not proven absent (inside lambda)
+    };
+    print("unreached\n");
+}
+
+main :: () -> s32 {
+    build();
+    return 0;
+}