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.

OSGKeyboardShared/Utilities/TranscriptLanguageDetector.swift

@@ -0,0 +1,44 @@
+// TranscriptLanguageDetector.swift
+// OSGKeyboard · Shared
+//
+// Lightweight script detection for choosing the language of LLM guidance.
+// This intentionally does not attempt full language identification.
+
+import Foundation
+
+public enum TranscriptLanguageDetector: Sendable {
+    /// Han characters as a share of non-whitespace, non-punctuation characters.
+    public static func cjkRatio(_ text: String) -> Double {
+        var hanCount = 0
+        var meaningfulCount = 0
+
+        for scalar in text.unicodeScalars {
+            if CharacterSet.whitespacesAndNewlines.contains(scalar)
+                || CharacterSet.punctuationCharacters.contains(scalar)
+                || CharacterSet.symbols.contains(scalar) {
+                continue
+            }
+            meaningfulCount += 1
+            if isHan(scalar) {
+                hanCount += 1
+            }
+        }
+
+        guard meaningfulCount > 0 else { return 0 }
+        return Double(hanCount) / Double(meaningfulCount)
+    }
+
+    /// Mixed Chinese/English transcripts should still receive Chinese guidance.
+    public static func prefersChineseGuidance(_ text: String) -> Bool {
+        cjkRatio(text) >= 0.15
+    }
+
+    private static func isHan(_ scalar: Unicode.Scalar) -> Bool {
+        switch scalar.value {
+        case 0x4E00...0x9FFF, 0x3400...0x4DBF, 0xF900...0xFAFF:
+            return true
+        default:
+            return false
+        }
+    }
+}