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/UtteranceTranscriptStitcherTests.swift

@@ -18,7 +18,7 @@ final class UtteranceTranscriptStitcherTests: XCTestCase {
         var stitcher = UtteranceTranscriptStitcher()
         stitcher.append(index: 1, text: "第二段")
         stitcher.append(index: 0, text: "第一段")
-        XCTAssertEqual(stitcher.composed(), "第一段 第二段")
+        XCTAssertEqual(stitcher.composed(), "第一段第二段")
     }
 
     func testComposedSafelyFallsBackWhenOverlapMergeShortensTooMuch() {
@@ -38,7 +38,21 @@ final class UtteranceTranscriptStitcherTests: XCTestCase {
         stitcher.append(index: 1, text: "第二段")
         stitcher.removeLastSegment()
         stitcher.append(index: 1, text: "第二段合并")
-        XCTAssertEqual(stitcher.composed(), "第一段 第二段合并")
+        XCTAssertEqual(stitcher.composed(), "第一段第二段合并")
+    }
+
+    func testComposedWithPauseMarksInsertsAboveThreshold() {
+        var stitcher = UtteranceTranscriptStitcher()
+        stitcher.append(index: 0, text: "第一段", trailingPauseSeconds: 0.8)
+        stitcher.append(index: 1, text: "第二段")
+        XCTAssertEqual(stitcher.composedWithPauseMarks(), "第一段 ⟨0.8s⟩ 第二段")
+    }
+
+    func testComposedSafelyRemainsMarkerFree() {
+        var stitcher = UtteranceTranscriptStitcher()
+        stitcher.append(index: 0, text: "第一段", trailingPauseSeconds: 0.8)
+        stitcher.append(index: 1, text: "第二段")
+        XCTAssertFalse(stitcher.composedSafely().contains("⟨"))
     }
 
     /// Documents the preMerge wipe hazard: append ignores empty text, so