lib/regex: pin exact loud-boundary text across all 11 deferred metachars (fold-2a review)

The loud-arm row asserted only a non-empty error on "a*" — a
half-ported arm returning any other error text, or another metachar
falling to the literal default, would have passed. Table over one
pattern per deferred arm ('^' leading, so the r_idx==0 skip gate
composes with the loud arm) compared against the exact boundary
text via strings.compare.

lib/regex/regex_test.ww

@@ -23,6 +23,7 @@ package regex;
 
 import regex;
 import os;
+import strings;
 
 let signalled: i32 = 0;
 fn fail() void = { os.exit(signalled + 10); };
@@ -244,13 +245,31 @@ fn fail() void = { os.exit(signalled + 10); };
 	};
 };
 
-// Any deferred metacharacter is a LOUD error — pins the fold-2a
-// boundary so the fold that ports '*' consciously deletes this row.
+// Every deferred metacharacter is a LOUD error carrying the exact
+// fold-boundary text — falling through to the literal default would
+// silently compile a wrong program, and any OTHER error text would
+// mean an arm was half-ported. One pattern per deferred arm so the
+// fold that ports an arm consciously deletes its row. '^' leads its
+// pattern: the r_idx==0 skip gate (regex.ha:261) must compose with
+// the loud arm, not bypass it.
 @test fn compile_metachar_loud() void = {
-	match (regex.compile("a*")) {
-	case let e: regex.error => { if ((e: str).len == 0) { fail(); }; };
+	let pats: [11]str = [
+		"a\\", "^a", "a$", "a[", "a(", "a)",
+		"a|", "a{", "a?", "a*", "a+",
+	];
+	let i: i32 = 0;
+	for (i < len(pats)) {
+		match (regex.compile(pats[i])) {
+		case let e: regex.error => {
+			if (strings.compare((e: str),
+				"regex: metacharacter not yet ported") != 0) {
+				fail();
+			};
+		};
 		case => fail();
 		};
+		i += 1;
+	};
 };
 
 export fn main() i32 = {