Minimal repro (issues/0073-...sx): a non-failable, uncalled closure literal
declared inside a `defer` body crashes the compiler with a SIGSEGV in
lowerLambda (src/ir/lower.zig). Isolation shows the trigger is "a closure
literal lowered inside a defer body" — not failability, not whether it's called
(closures and failable closures lower fine outside a defer). Pre-existing
lowering bug, unrelated to the A5 error-analysis extraction; surfaced while
writing an A5.2 cleanup-absorption test example.
Filed per the IMPASSIBLE RULE: work paused pending a fix in another session.
