selfhost: mirror defer; e2e tests for LIFO ordering

C cgen has carried defer for a while (defers[] global + reverse
walk on every return). Selfhost cgen now mirrors:

- cgen struct: deferbuf (**node, LIFO stack) + defertop counter.
- cgstmt N_DEFER: push n.lhs.
- cgreturn: rundefers() at entry — same as the C cgen pattern.
- cgfn fall-through return: rundefers() before zero-AX+RET.

DEFER_MAX = 16 matches C cgen.

Two new e2e rows: defer with an explicit `return acc;` (321 mod 256
= 65), and defer firing on an implicit void-fn fall-through (87).
Both rows verified via the wwstage cgen too.

Defer's semantics: queued exprs fire LIFO before the return expr
is evaluated, so a return that reads memory mutated by a deferred
call sees the post-defer state. Matches C cgen and Hare.

selfhost/cmd/w6c/main.combined.ww

@@ -6542,6 +6542,14 @@ fn cgstmt(c: *cgen, n: *node) void = {
 
 	if (k == N_YIELD) { cgyield(c, n); return; };
 
+	if (k == N_DEFER) {
+		if (c.defertop < DEFER_MAX) {
+			c.deferbuf[c.defertop] = n.lhs;
+			c.defertop += 1;
+		};
+		return;
+	};
+
 	c.lastwasreturn = 0;
 };
 
@@ -6569,7 +6577,19 @@ fn cgblock(c: *cgen, n: *node) void = {
 	return;
 };
 
+// rundefers — emit cgexpr for every queued defer in LIFO order.
+// Called from cgreturn and the cgfn implicit-return path.
+fn rundefers(c: *cgen) void = {
+	let i: i32 = c.defertop - 1;
+	for (i >= 0) {
+		cgexpr(c, c.deferbuf[i]);
+		i -= 1;
+	};
+	return;
+};
+
 fn cgreturn(c: *cgen, n: *node) void = {
+	rundefers(c);
 	let rhs: *node = n.lhs;
 	if (rhs != nil) {
 		// Tuple return `return a, b;`:
@@ -7463,6 +7483,9 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	if (fn_.body != nil) { cgstmt(c, fn_.body); };
 
 	if (c.lastwasreturn == 0) {
+		// Run any registered defers in LIFO order before the
+		// implicit return.
+		rundefers(c);
 		// Zero AX before the fall-through return — matches C cgen,
 		// which always emits this so void-returning fns don't leak
 		// a stale callee value to their caller.
@@ -7646,6 +7669,7 @@ type ffi = struct {
 };
 
 def LOOP_MAX: i32 = 16;
+def DEFER_MAX: i32 = 16;
 
 type cgen = struct {
 	a:        *arena,
@@ -7668,6 +7692,8 @@ type cgen = struct {
 	loopcontbuf: *str,	// stack of cont labels for continue
 	yieldtop: i32,
 	yieldbuf:    *str,	// stack of match end labels for yield
+	defertop:    i32,
+	deferbuf:    **node,	// stack of deferred exprs (LIFO at return)
 };
 
 fn cgeninit(c: *cgen, a: *arena) void = {
@@ -7684,6 +7710,8 @@ fn cgeninit(c: *cgen, a: *arena) void = {
 	c.loopcontbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
 	c.yieldtop = 0;
 	c.yieldbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
+	c.defertop = 0;
+	c.deferbuf = amalloc(a, (DEFER_MAX: u64) * 8u64): **node;
 };
 
 // localalloc — append a slot for `name` without dedup. Used for

selfhost/cmd/wcc/cgen.ww

@@ -145,6 +145,7 @@ type ffi = struct {
 };
 
 def LOOP_MAX: i32 = 16;
+def DEFER_MAX: i32 = 16;
 
 type cgen = struct {
 	a:        *arena,
@@ -167,6 +168,8 @@ type cgen = struct {
 	loopcontbuf: *str,	// stack of cont labels for continue
 	yieldtop: i32,
 	yieldbuf:    *str,	// stack of match end labels for yield
+	defertop:    i32,
+	deferbuf:    **node,	// stack of deferred exprs (LIFO at return)
 };
 
 fn cgeninit(c: *cgen, a: *arena) void = {
@@ -183,6 +186,8 @@ fn cgeninit(c: *cgen, a: *arena) void = {
 	c.loopcontbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
 	c.yieldtop = 0;
 	c.yieldbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
+	c.defertop = 0;
+	c.deferbuf = amalloc(a, (DEFER_MAX: u64) * 8u64): **node;
 };
 
 // localalloc — append a slot for `name` without dedup. Used for

selfhost/cmd/wcc/cgendecl.ww

@@ -258,6 +258,9 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	if (fn_.body != nil) { cgstmt(c, fn_.body); };
 
 	if (c.lastwasreturn == 0) {
+		// Run any registered defers in LIFO order before the
+		// implicit return.
+		rundefers(c);
 		// Zero AX before the fall-through return — matches C cgen,
 		// which always emits this so void-returning fns don't leak
 		// a stale callee value to their caller.

selfhost/cmd/wcc/cgenstmt.ww

@@ -43,6 +43,14 @@ fn cgstmt(c: *cgen, n: *node) void = {
 
 	if (k == N_YIELD) { cgyield(c, n); return; };
 
+	if (k == N_DEFER) {
+		if (c.defertop < DEFER_MAX) {
+			c.deferbuf[c.defertop] = n.lhs;
+			c.defertop += 1;
+		};
+		return;
+	};
+
 	c.lastwasreturn = 0;
 };
 
@@ -70,7 +78,19 @@ fn cgblock(c: *cgen, n: *node) void = {
 	return;
 };
 
+// rundefers — emit cgexpr for every queued defer in LIFO order.
+// Called from cgreturn and the cgfn implicit-return path.
+fn rundefers(c: *cgen) void = {
+	let i: i32 = c.defertop - 1;
+	for (i >= 0) {
+		cgexpr(c, c.deferbuf[i]);
+		i -= 1;
+	};
+	return;
+};
+
 fn cgreturn(c: *cgen, n: *node) void = {
+	rundefers(c);
 	let rhs: *node = n.lhs;
 	if (rhs != nil) {
 		// Tuple return `return a, b;`:

selfhost/cmd/wwdump/main.combined.ww

@@ -6542,6 +6542,14 @@ fn cgstmt(c: *cgen, n: *node) void = {
 
 	if (k == N_YIELD) { cgyield(c, n); return; };
 
+	if (k == N_DEFER) {
+		if (c.defertop < DEFER_MAX) {
+			c.deferbuf[c.defertop] = n.lhs;
+			c.defertop += 1;
+		};
+		return;
+	};
+
 	c.lastwasreturn = 0;
 };
 
@@ -6569,7 +6577,19 @@ fn cgblock(c: *cgen, n: *node) void = {
 	return;
 };
 
+// rundefers — emit cgexpr for every queued defer in LIFO order.
+// Called from cgreturn and the cgfn implicit-return path.
+fn rundefers(c: *cgen) void = {
+	let i: i32 = c.defertop - 1;
+	for (i >= 0) {
+		cgexpr(c, c.deferbuf[i]);
+		i -= 1;
+	};
+	return;
+};
+
 fn cgreturn(c: *cgen, n: *node) void = {
+	rundefers(c);
 	let rhs: *node = n.lhs;
 	if (rhs != nil) {
 		// Tuple return `return a, b;`:
@@ -7463,6 +7483,9 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	if (fn_.body != nil) { cgstmt(c, fn_.body); };
 
 	if (c.lastwasreturn == 0) {
+		// Run any registered defers in LIFO order before the
+		// implicit return.
+		rundefers(c);
 		// Zero AX before the fall-through return — matches C cgen,
 		// which always emits this so void-returning fns don't leak
 		// a stale callee value to their caller.
@@ -7646,6 +7669,7 @@ type ffi = struct {
 };
 
 def LOOP_MAX: i32 = 16;
+def DEFER_MAX: i32 = 16;
 
 type cgen = struct {
 	a:        *arena,
@@ -7668,6 +7692,8 @@ type cgen = struct {
 	loopcontbuf: *str,	// stack of cont labels for continue
 	yieldtop: i32,
 	yieldbuf:    *str,	// stack of match end labels for yield
+	defertop:    i32,
+	deferbuf:    **node,	// stack of deferred exprs (LIFO at return)
 };
 
 fn cgeninit(c: *cgen, a: *arena) void = {
@@ -7684,6 +7710,8 @@ fn cgeninit(c: *cgen, a: *arena) void = {
 	c.loopcontbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
 	c.yieldtop = 0;
 	c.yieldbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
+	c.defertop = 0;
+	c.deferbuf = amalloc(a, (DEFER_MAX: u64) * 8u64): **node;
 };
 
 // localalloc — append a slot for `name` without dedup. Used for