feat(polish): add context safeguards, layered prompts, and output validation

Use redacted cursor neighborhood and pause-aware chunks for more natural polish,
validate protected terms with retry/local fallback, and structure bilingual prompts
for consistency and provider prefix caching.

OSGKeyboardTests/TranscriptLanguageDetectorTests.swift

@@ -0,0 +1,20 @@
+import XCTest
+@testable import OSGKeyboardShared
+
+final class TranscriptLanguageDetectorTests: XCTestCase {
+    func testChineseAndMixedInputPreferChineseGuidance() {
+        XCTAssertTrue(TranscriptLanguageDetector.prefersChineseGuidance("今天开会讨论 roadmap"))
+        XCTAssertTrue(TranscriptLanguageDetector.prefersChineseGuidance("把 PRD 发给 Ali review"))
+    }
+
+    func testEnglishJapaneseAndKoreanDoNotPreferChineseGuidance() {
+        XCTAssertFalse(TranscriptLanguageDetector.prefersChineseGuidance("ship it tomorrow"))
+        XCTAssertFalse(TranscriptLanguageDetector.prefersChineseGuidance("こんにちは"))
+        XCTAssertFalse(TranscriptLanguageDetector.prefersChineseGuidance("안녕하세요"))
+    }
+
+    func testNumbersHaveNoScriptSignal() {
+        XCTAssertEqual(TranscriptLanguageDetector.cjkRatio("12345"), 0)
+        XCTAssertEqual(TranscriptLanguageDetector.cjkRatio(""), 0)
+    }
+}