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.

test/wcc/847_const_reassign_reject.c

@@ -0,0 +1,173 @@
+/*
+ * 847_const_reassign_reject (catB-22) — cstage and wwstage LOUD-REJECT
+ * a reassignment of a `const`-declared binding.
+ *
+ * `const x = 5; x = 6;` mutates an immutable binding — a programmer
+ * error. The sym carries an is_const flag (lib/ww/sym.ww), but wwstage
+ * never SET it at the let-install nor CONSUMED it at assignment, so the
+ * reassignment slipped through silently. The fix mirrors 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 UP.
+ *
+ * Both rows annotate `: i32` so the inferred-int default (#34 int/i32
+ * cs/ww divergence) does not contaminate the const signal.
+ *
+ *   row        | shape                          | expect
+ *   -----------+--------------------------------+--------
+ *   const_set  | const x: i32 = 5; x = 6;       | REJECT
+ *   let_set    | let y: i32 = 5; y = 6;         | BUILD, run 6
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return -1;
+}
+
+static void
+outbin(char *dst, size_t n, const char *tmpdir, const char *src)
+{
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	snprintf(dst, n, "%s/%s", tmpdir, base);
+	char *dot = strrchr(dst, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+}
+
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128];
+	snprintf(s, sizeof s, "/tmp/creas_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/creas_%d_d_%d", getpid(), i);
+	FILE *f = fopen(s, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, s);
+	int rc = runwait(cmd);
+	outbin(bin, sizeof bin, tmpdir, s);
+	unlink(s);
+	unlink(bin);
+	rmdir(tmpdir);
+	if (rc == 0)
+		fprintf(stderr, "creas[%s][%s]: built ok, expected a reject\n",
+		    driver, label);
+	return rc == 0 ? -1 : 0;
+}
+
+static int
+run_build(const char *driver, const char *label, const char *src, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128];
+	snprintf(s, sizeof s, "/tmp/creas_ok_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/creas_ok_%d_d_%d", getpid(), i);
+	FILE *f = fopen(s, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, s);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "creas[%s][%s]: build failed (a mutable let "
+		    "must compile)\n", driver, label);
+		unlink(s); rmdir(tmpdir);
+		return -1;
+	}
+	outbin(bin, sizeof bin, tmpdir, s);
+	int got = runwait(bin);
+	unlink(s); unlink(bin); rmdir(tmpdir);
+	return got;
+}
+
+struct rejrow { const char *label; const char *src; };
+
+static const struct rejrow reject_rows[] = {
+	{ "const_set",
+	  "package main;\n"
+	  "fn main() i32 = { const x: i32 = 5; x = 6; return x; };\n" },
+};
+
+struct okrow { const char *label; const char *src; int want; };
+
+static const struct okrow ok_rows[] = {
+	{ "let_set",
+	  "package main;\n"
+	  "fn main() i32 = { let y: i32 = 5; y = 6; return y; };\n", 6 },
+};
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[1024], wdrv[1024];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *path; int gated; } drivers[] = {
+		{ "cstage",  cdrv, 0 },
+		{ "wwstage", wdrv, 1 },
+		{ NULL, NULL, 0 },
+	};
+
+	int nrej = (int)(sizeof reject_rows / sizeof reject_rows[0]);
+	int nok = (int)(sizeof ok_rows / sizeof ok_rows[0]);
+	int total = 0, fail = 0;
+
+	for (int d = 0; drivers[d].name; d++) {
+		if (drivers[d].gated && access(drivers[d].path, X_OK) != 0) {
+			fprintf(stderr, "creas: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < nrej; i++) {
+			total++;
+			if (build_should_fail(drivers[d].path,
+			    reject_rows[i].label, reject_rows[i].src,
+			    d * 100 + i) != 0)
+				fail++;
+		}
+		for (int i = 0; i < nok; i++) {
+			total++;
+			int got = run_build(drivers[d].path, ok_rows[i].label,
+			    ok_rows[i].src, d * 100 + i);
+			if (got != ok_rows[i].want) {
+				fprintf(stderr, "creas[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, ok_rows[i].label,
+				    got, ok_rows[i].want);
+				fail++;
+			}
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr, "creas: %d/%d fixtures failed\n", fail, total);
+		return 1;
+	}
+	printf("creas: %d/%d ok\n", total, total);
+	return 0;
+}