wcc/ww: reject reassignment of a const binding (catB-22)

The sym carried an is_const flag (lib/ww/sym.ww) but wwstage never
set it at the let-install nor consumed it at assignment, so mutating
a `const` slipped through silently. Mirror cstage's two sites: set
is_const when n.op == TK_CONST at the local let-install (cmd/wcc/
check.c:2408) and reject an N_ASSIGN whose lhs ident resolves to an
is_const sym (cmd/wcc/check.c:1889-1896). cstage already rejected;
this aligns wwstage's w6c_ww UP. A bare `_` discard lvalue (empty
str) is skipped.

Regen w6c/wwdump combined.ww (checker embeds in both). Valid-program
codegen unchanged → cs==ww byte-id gate stays green.

selfhost/cmd/w6c/main.combined.ww

@@ -11247,7 +11247,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -15947,6 +15954,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store

selfhost/cmd/wcc/check.ww

@@ -773,7 +773,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -5473,6 +5480,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store

selfhost/cmd/wwdump/main.combined.ww

@@ -11247,7 +11247,14 @@ fn resolvewalk(c: *checker, n: *node) void = {
 		let nm: str = n.str;
 		if (nm.len > 0) {
 			checkmoduleshadow(c, nm, "let");
-			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			let s: *sym = scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
+			// catB-22: flag a `const` binding so checkassign can
+			// reject a later reassignment. The parser stamps
+			// n.op = TK_CONST (parse/stmt.ww). Mirror cstage
+			// cmd/wcc/check.c:2408.
+			if (s != nil) {
+				if (n.op == tkind.TK_CONST) { s.is_const = 1i32; };
+			};
 		};
 	};
 
@@ -15947,6 +15954,21 @@ fn checkassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
 	if (n.rhs == nil) { return; };
+	// catB-22: reject reassigning a `const`-flagged binding (the
+	// is_const bit set at the let-install above). A bare `_` discard
+	// lvalue carries an empty str and is skipped. Mirror cstage
+	// cmd/wcc/check.c:1889-1896.
+	if (n.lhs.kind == nkind.N_IDENT) {
+		if (n.lhs.str.len > 0) {
+			let s: *sym = scopelookupprefer(c.cur, c.curmod, n.lhs.str);
+			if (s != nil) {
+				if (s.is_const != 0i32) {
+					cerr("error: cannot assign to const binding\n");
+					c.errs += 1;
+				};
+			};
+		};
+	};
 	let ltn: *node = exprtype(c, n.lhs, nil);
 	let rtn: *node = exprtype(c, n.rhs, nil);
 	// #29: a rune literal narrowing into an integer assign/index-store