fix(ir): open a fresh defer window when lowering a lambda body (issue 0073)

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.

src/ir/lower.zig

@@ -8313,6 +8313,19 @@ pub const Lowering = struct {
         defer self.current_ctx_ref = saved_ctx_ref_lam;
         if (lambda_wants_ctx) self.current_ctx_ref = Ref.fromIndex(0);
 
+        // A lambda is its own function: its `return` must drain only ITS OWN
+        // `defer`s, not the enclosing function's. Open a fresh defer window
+        // (like `lowerFunction`/`monomorphizeFunction`) and restore on exit —
+        // otherwise lowering a closure literal inside a `defer` body re-enters
+        // the enclosing function's defer drain (infinite recursion — issue 0073).
+        const saved_func_defer_base = self.func_defer_base;
+        const saved_defer_len = self.defer_stack.items.len;
+        defer {
+            self.func_defer_base = saved_func_defer_base;
+            self.defer_stack.shrinkRetainingCapacity(saved_defer_len);
+        }
+        self.func_defer_base = saved_defer_len;
+
         // Create entry block
         const entry_name = self.module.types.internString("entry");
         const entry = self.builder.appendBlock(entry_name, &.{});