A closure literal declared inside a `defer` body segfaulted the compiler.
Root cause: lowerLambda never opened its own `func_defer_base` window. Every
other function-lowering entry (lowerFunction / monomorphizeFunction /
monomorphizePackFn) saves func_defer_base, sets it to defer_stack.items.len, and
restores it — lowerLambda didn't. So a lambda's `return` drained the ENCLOSING
function's defers; when the defer body itself declared the lambda, draining
re-lowered the lambda, which returned, which drained again → infinite recursion
→ stack-overflow SIGSEGV (the failable variant surfaced one frame out, in
expandCallDefaults→lookupFn reading a clobbered scope).
Fix: lowerLambda now saves func_defer_base + the defer_stack length, sets the
base to the current length (a fresh window), and restores both on exit — so a
lambda's `return` drains only its own defers.
Regression: examples/0310-closures-closure-literal-in-defer.sx — a closure
declared and called inside a `defer`; verifies `body` then `defer closure: 42`
at scope exit (exit 0). Issue 0073 marked RESOLVED; repro promoted from
issues/0073-*.sx.
zig build, zig build test, tests/run_examples.sh (358/0) all green.
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.
