feat: add vocabulary-aware Transcriber and shared correction/result types

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

Tests/SstCoreTests/TranscriberVocabularyTests.swift

@@ -0,0 +1,21 @@
+import Foundation
+import Testing
+@testable import SstCore
+
+private struct PlainTranscriber: Transcriber {
+    func transcribe(_ samples: [Float]) async throws -> String { "plain \(samples.count)" }
+}
+
+struct TranscriberVocabularyTests {
+    @Test func vocabularyVariantFallsBackToPlainTranscribe() async throws {
+        let result = try await PlainTranscriber().transcribe([0, 0, 0], vocabulary: ["Anthropic"])
+        #expect(result == "plain 3")
+    }
+
+    @Test func appliedCorrectionRoundTripsThroughJSON() throws {
+        let original = AppliedCorrection(heard: "cloud code", wrote: "Claude Code", rule: "cloud code", start: 12, length: 11)
+        let data = try JSONEncoder().encode(original)
+        let decoded = try JSONDecoder().decode(AppliedCorrection.self, from: data)
+        #expect(decoded == original)
+    }
+}