Stop a proof-of-work test failing once every few thousand runs

"an unsolved nonce is refused" issued a fresh challenge, then asserted
that the literal nonce "definitely-not-it" does not satisfy 12 bits of
work. A challenge carries 128 random bits, so that literal is not an
unsolved nonce; it is a nonce with a 1-in-4096 chance of being the
answer. Measured over 200,000 sampled challenges: 1 in 4,300.

It came due today, on a documentation-only merge request, where a red
`test` job is impossible to read as anything but a real break. It also
sat in front of this branch on a single shared runner.

The test now picks a nonce that demonstrably does not solve the
challenge it was actually given, so it tests what its name says without
depending on luck.

It rides along with the change beside it because it can fail that
change's own pipeline, and the runner these queue on takes them one at
a time.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Tests/FlamenetRelayTests/SignupProofOfWorkTests.swift

@@ -26,11 +26,22 @@ struct SignupProofOfWorkTests {
     }
 
     @Test("an unsolved nonce is refused")
-    func unsolvedRefused() {
+    func unsolvedRefused() throws {
         let issued = SignupChallenge.issue(secret: secret, now: now, bits: 12)
+        // A challenge carries 128 random bits, so a nonce fixed in the source
+        // is not an unsolved nonce, it is a nonce with a 1-in-4096 chance of
+        // being the solution. That is a test which fails on its own every few
+        // thousand runs, and it did. Pick one that demonstrably does not solve
+        // THIS challenge instead.
+        let unsolved = try #require(
+            (0..<64).map(String.init).first { candidate in
+                SignupChallenge.leadingZeroBits(
+                    of: SHA256.hash(
+                        data: Data("\(issued.challenge).\(candidate)".utf8))) < 12
+            })
         #expect(throws: SignupChallenge.Refusal.insufficient) {
             try SignupChallenge.verify(
-                challenge: issued.challenge, nonce: "definitely-not-it",
+                challenge: issued.challenge, nonce: unsolved,
                 secret: self.secret, now: self.now, bits: 12)
         }
     }