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/Services/LLMCacheMetricsStore.swift

@@ -0,0 +1,51 @@
+// LLMCacheMetricsStore.swift
+// OSGKeyboard · Shared
+//
+// Small App Group diagnostic snapshot for validating provider prompt caching.
+
+import Foundation
+
+public struct LLMCacheMetrics: Codable, Equatable, Sendable {
+    public let providerId: String
+    public let promptTokens: Int?
+    public let cachedTokens: Int?
+    public let observedAt: TimeInterval
+
+    public var summary: String {
+        guard let cachedTokens else { return "n/a (\(providerId))" }
+        guard let promptTokens, promptTokens > 0 else {
+            return "\(cachedTokens) cached (\(providerId))"
+        }
+        let rate = Int((Double(cachedTokens) / Double(promptTokens) * 100).rounded())
+        return "\(cachedTokens)/\(promptTokens) \(rate)% (\(providerId))"
+    }
+}
+
+public enum LLMCacheMetricsStore {
+    private static let key = "debug.llmCacheMetrics.v1"
+
+    public static func record(
+        providerId: String,
+        promptTokens: Int?,
+        cachedTokens: Int?,
+        defaults: UserDefaults? = nil
+    ) {
+        guard let store = defaults ?? AppGroup.defaultsIfAvailable else { return }
+        let metrics = LLMCacheMetrics(
+            providerId: providerId.isEmpty ? "openai-compatible" : providerId,
+            promptTokens: promptTokens,
+            cachedTokens: cachedTokens,
+            observedAt: Date().timeIntervalSince1970
+        )
+        guard let data = try? JSONEncoder().encode(metrics) else { return }
+        store.set(data, forKey: key)
+    }
+
+    public static func latest(defaults: UserDefaults? = nil) -> LLMCacheMetrics? {
+        guard let store = defaults ?? AppGroup.defaultsIfAvailable,
+              let data = store.data(forKey: key) else {
+            return nil
+        }
+        return try? JSONDecoder().decode(LLMCacheMetrics.self, from: data)
+    }
+}