feat: add SstData with DictionaryFile and word knowledge

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

Tests/SstDataTests/Fakes.swift

@@ -0,0 +1,13 @@
+import Foundation
+@testable import SstData
+
+/// Deterministic stand-in for the macOS spell checker.
+struct FakeKnowledge: WordKnowledge {
+    var words: Set<String> = ["cloud", "code", "some", "thing", "something", "reef", "slack", "in", "de", "the", "of"]
+    var functionWords: Set<String> = ["in", "de", "the", "of"]
+    func isWord(_ word: String) -> Bool { words.contains(word.lowercased()) }
+    func isFunctionWord(_ word: String) -> Bool { functionWords.contains(word.lowercased()) }
+}
+
+func term(_ text: String) -> DictionaryEntry { DictionaryEntry(kind: .term(text)) }
+func pair(_ heard: String, _ wrote: String) -> DictionaryEntry { DictionaryEntry(kind: .correction(heard: heard, wrote: wrote)) }