test/926: pin #38b/#40 loud-stop markers + 5 more consumption-shape rows (#38 review)

Buildfail rows previously accepted ANY non-zero compiler exit — an
unrelated checker error would masquerade as loud-stop coverage; now
the stderr must carry the #38b/#40 marker on BOTH stages. New rows:
wide_discard_stmt (generic @sretscr discard, frame canary) and four
loud-stop shapes verified by probe during review — `?` consumption,
`is` consumption, tagged-GLOBAL receive, struct-literal tagged field
from an sret-class call (the widener #40 gate).

test/wcc/926_tagged_sret_run.c

@@ -356,18 +356,96 @@ static const struct row rows[] = {
 	  "    return 0;\n"
 	  "};\n",
 	  0, 0, 1 },
+	/* Discard statement: rides the generic @sretscr slot; the frame
+	 * canary pins no clobber across two discarded wide results. */
+	{ "wide_discard_stmt",
+	  WIDE_TYPES
+	  "fn mk(b: []u8) (wide | oops | nomem) = {\n"
+	  "    return wide { xs = b, ys = b, n = 1 };\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "    let canary: i64 = 777;\n"
+	  "    let buf: [4]u8 = [1u8, 2u8, 3u8, 4u8];\n"
+	  "    mk(buf[0:4]);\n"
+	  "    mk(buf[0:4]);\n"
+	  "    if (canary != 777) { return 1; };\n"
+	  "    return 0;\n"
+	  "};\n",
+	  0, 1, 0 },
+	/* LOUD-STOP: `?` consuming an sret-class call result (cstage
+	 * gates at the N_LET TRYUNW/TRYPROP shape, wwstage at cgtryprop
+	 * — same acceptance, rule 10). */
+	{ "fail_tryprop_wide",
+	  WIDE_TYPES
+	  "fn mk(b: []u8) (wide | oops | nomem) = {\n"
+	  "    return wide { xs = b, ys = b, n = 1 };\n"
+	  "};\n"
+	  "fn go(b: []u8) (i64 | oops | nomem) = {\n"
+	  "    let w: wide = mk(b)?;\n"
+	  "    return w.n;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [4]u8 = [1u8, 2u8, 3u8, 4u8];\n"
+	  "    match (go(buf[0:4])) {\n"
+	  "    case let v: i64 => { if (v != 1) { return 1; }; };\n"
+	  WIDE_MAIN_CHECK,
+	  0, 0, 1 },
+	/* LOUD-STOP: `is` on an sret-class call result (AX = dest
+	 * pointer, not the tag). */
+	{ "fail_typetest_wide",
+	  WIDE_TYPES
+	  "fn mk(b: []u8) (wide | oops | nomem) = {\n"
+	  "    return wide { xs = b, ys = b, n = 1 };\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [4]u8 = [1u8, 2u8, 3u8, 4u8];\n"
+	  "    if (mk(buf[0:4]) is wide) { return 0; };\n"
+	  "    return 1;\n"
+	  "};\n",
+	  0, 0, 1 },
+	/* LOUD-STOP: sret receive into a tagged GLOBAL lvalue. */
+	{ "fail_global_receive",
+	  WIDE_TYPES
+	  "let g: (wide | oops | nomem);\n"
+	  "fn mk(b: []u8) (wide | oops | nomem) = {\n"
+	  "    return wide { xs = b, ys = b, n = 1 };\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [4]u8 = [1u8, 2u8, 3u8, 4u8];\n"
+	  "    g = mk(buf[0:4]);\n"
+	  "    match (g) {\n"
+	  "    case let w: wide => { if (w.n != 1) { return 1; }; };\n"
+	  WIDE_MAIN_CHECK,
+	  0, 0, 1 },
+	/* LOUD-STOP: struct-literal FIELD of tagged type from an
+	 * sret-class call — the widener's cursor-spill #40 gate. */
+	{ "fail_structfield_widen",
+	  WIDE_TYPES
+	  "type holder = struct { r: (wide | oops | nomem), k: i64 };\n"
+	  "fn mk(b: []u8) (wide | oops | nomem) = {\n"
+	  "    return wide { xs = b, ys = b, n = 1 };\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "    let buf: [4]u8 = [1u8, 2u8, 3u8, 4u8];\n"
+	  "    let h: holder = holder { r = mk(buf[0:4]), k = 5 };\n"
+	  "    match (h.r) {\n"
+	  "    case let w: wide => { if (w.n != 1) { return 1; }; };\n"
+	  WIDE_MAIN_CHECK,
+	  0, 0, 1 },
 };
 
 static const char *g_bin;
 
-/* compile one row with `tool` (w6c or w6c_ww) into outpath; returns
+/* compile one row with `tool` (w6c or w6c_ww) into outpath; stderr
- * the tool's exit code. */
+ * goes to errpath so buildfail rows can pin the loud-stop marker.
+ * Returns the tool's exit code. */
 static int
-compile_s(const char *tool, const char *src, const char *outpath)
+compile_s(const char *tool, const char *src, const char *outpath,
+    const char *errpath)
 {
 	char cmd[1024];
-	snprintf(cmd, sizeof cmd, "%s/%s %s > %s 2>/dev/null",
+	snprintf(cmd, sizeof cmd, "%s/%s %s > %s 2> %s",
-	    g_bin, tool, src, outpath);
+	    g_bin, tool, src, outpath, errpath);
 	return runwait(cmd);
 }
 
@@ -435,20 +513,25 @@ main(void)
 	for (int i = 0; i < n; i++) {
 		const struct row *r = &rows[i];
 		char src[128], cs_s[128], ww_s[128];
+		char cs_e[128], ww_e[128];
 		snprintf(src, sizeof src, "/tmp/tsret_%d_%d.ww",
 		    getpid(), i);
 		snprintf(cs_s, sizeof cs_s, "/tmp/tsret_%d_%d_cs.s",
 		    getpid(), i);
 		snprintf(ww_s, sizeof ww_s, "/tmp/tsret_%d_%d_ww.s",
 		    getpid(), i);
+		snprintf(cs_e, sizeof cs_e, "/tmp/tsret_%d_%d_cs.err",
+		    getpid(), i);
+		snprintf(ww_e, sizeof ww_e, "/tmp/tsret_%d_%d_ww.err",
+		    getpid(), i);
 		FILE *f = fopen(src, "wb");
 		if (!f) return 1;
 		fputs("package main;\n\n", f);
 		fputs(r->src, f);
 		fclose(f);
 
-		int cs_rc = compile_s("w6c", src, cs_s);
+		int cs_rc = compile_s("w6c", src, cs_s, cs_e);
-		int ww_rc = compile_s("w6c_ww", src, ww_s);
+		int ww_rc = compile_s("w6c_ww", src, ww_s, ww_e);
 		if (r->buildfail) {
 			total++;
 			if (cs_rc == 0 || ww_rc == 0) {
@@ -456,8 +539,18 @@ main(void)
 				    "expected, cstage rc=%d wwstage rc=%d\n",
 				    r->label, cs_rc, ww_rc);
 				fail++;
+			} else if (!(file_has(cs_e, "#38b") || file_has(cs_e, "#40"))
+			    || !(file_has(ww_e, "#38b") || file_has(ww_e, "#40"))) {
+				/* the rejection must be THE #38b/#40 loud-stop,
+				 * not an unrelated checker error masquerading
+				 * as coverage. */
+				fprintf(stderr, "FAIL row[%s]: rejected but "
+				    "without the #38b/#40 loud-stop marker\n",
+				    r->label);
+				fail++;
 			}
 			unlink(src); unlink(cs_s); unlink(ww_s);
+			unlink(cs_e); unlink(ww_e);
 			continue;
 		}
 		total++;
@@ -466,6 +559,7 @@ main(void)
 			    "ww=%d\n", r->label, cs_rc, ww_rc);
 			fail++;
 			unlink(src); unlink(cs_s); unlink(ww_s);
+			unlink(cs_e); unlink(ww_e);
 			continue;
 		}
 		if (!file_eq(cs_s, ww_s)) {
@@ -505,6 +599,7 @@ main(void)
 			}
 		}
 		unlink(src); unlink(cs_s); unlink(ww_s);
+		unlink(cs_e); unlink(ww_e);
 	}
 	if (fail) {
 		fprintf(stderr, "tagged_sret_run: %d/%d rows failed\n",