feat: controller supports vocabulary bias, prompt-echo guard, corrector and result hook

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Sources/SstCore/PromptEcho.swift

@@ -0,0 +1,21 @@
+import Foundation
+
+/// Whisper sometimes answers a vocabulary prompt with the prompt itself when there is little speech.
+public enum PromptEcho {
+    static func normalize(_ text: String) -> String {
+        String(text.lowercased().map { $0.isLetter || $0.isNumber ? $0 : " " })
+            .split(separator: " ")
+            .joined(separator: " ")
+    }
+
+    /// True when `text` is nothing but the first k ≥ 2 terms of the prompt.
+    /// A single term on its own is valid speech ("Anthropic").
+    public static func isEcho(_ text: String, vocabulary: [String]) -> Bool {
+        guard vocabulary.count >= 2 else { return false }
+        let normalized = normalize(text)
+        guard !normalized.isEmpty else { return false }
+        return (2...vocabulary.count).contains { k in
+            normalize(vocabulary.prefix(k).joined(separator: " ")) == normalized
+        }
+    }
+}