Ciphers use fire-and-forget File.WriteAllTextAsync (unawaited)

[SimAin]

Summary

Every cipher writes its output with File.WriteAllTextAsync(...) but never awaits the returned Task (the enclosing encode/decode methods are synchronous void). This is a fire-and-forget call: the write may not have flushed by the time the method returns, and any I/O exception is swallowed silently. The synchronous File.WriteAllText(...) should be used instead.

Why it's a bug

• Race / data loss: the process can continue (or exit) before the async write completes. This is exactly the kind of round-trip (encode → immediately decode) the app performs; results can read stale or partial output.
• Swallowed errors: an exception in the unawaited Task (e.g. file locked, permissions) is never observed — the user sees a "success" message while nothing was written.
• No async benefit is gained anyway, since the call sites are synchronous and single-threaded.

Affected locations (12 calls across 6 ciphers)

• cryptography/HistoricCiphers/CaesarCipher.cs:43, 50
• cryptography/HistoricCiphers/VigenereCipher.cs:48, 56
• cryptography/HistoricCiphers/SimpleSubCipher.cs:48, 56
• cryptography/HistoricCiphers/SimpleTranspositionCipher.cs:46, 55
• cryptography/HistoricCiphers/RailFenceCipher.cs:49, 63
• cryptography/HistoricCiphers/PlayfairCipher.cs:59, 74

Proposed fix

Replace File.WriteAllTextAsync(writeToFile, output) with the synchronous File.WriteAllText(writeToFile, output) at all call sites. Simplest, correct, and matches the synchronous nature of these methods — no async/await plumbing needed for a single-threaded console app.

Acceptance criteria

• All cipher writes are synchronous and complete before the method returns
• No unawaited Task / fire-and-forget calls remain (grep WriteAllTextAsync returns nothing in HistoricCiphers/)
• dotnet build clean; all tests pass
• Encode → decode round-trip produces correct output reliably