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.

examples/0310-closures-closure-literal-in-defer.sx

@@ -0,0 +1,22 @@
+// Closure literal declared (and used) inside a `defer` body.
+//
+// Regression (issue 0073): this used to segfault lowering. A lambda inherited
+// the enclosing function's `func_defer_base`, so the lambda's `return` re-drained
+// the enclosing function's defers — and when the defer body itself declared the
+// lambda, that re-lowered the lambda forever (infinite recursion). A lambda now
+// opens its own defer window (like every other function-lowering entry).
+
+#import "modules/std.sx";
+
+run :: () {
+    defer {
+        cb := (n: s32) -> s32 { return n * 2; };
+        print("defer closure: {}\n", cb(21));   // 42, at scope exit
+    }
+    print("body\n");
+}
+
+main :: () -> s32 {
+    run();
+    return 0;
+}

examples/expected/0310-closures-closure-literal-in-defer.exit

@@ -0,0 +1 @@
+0

examples/expected/0310-closures-closure-literal-in-defer.stderr

@@ -0,0 +1 @@
+

examples/expected/0310-closures-closure-literal-in-defer.stdout

@@ -0,0 +1,2 @@
+body
+defer closure: 42