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/PolishOutputValidatorTests.swift

@@ -0,0 +1,51 @@
+import XCTest
+@testable import OSGKeyboardShared
+
+final class PolishOutputValidatorTests: XCTestCase {
+    func testMissingDictionaryCanonicalTermIsHardViolation() {
+        let dictionary = PersonalDictionary(entries: [
+            .init(
+                term: "Kubernetes",
+                aliases: ["k8s"],
+                category: .productName,
+                source: .manual
+            ),
+        ])
+        let violations = PolishOutputValidator.validate(
+            input: "部署 k8s 集群",
+            output: "部署容器集群",
+            dictionary: dictionary,
+            lengthRatio: 0.5...2
+        )
+        XCTAssertTrue(violations.contains(.missingDictionaryTerms(["Kubernetes"])))
+        XCTAssertTrue(violations.contains(where: \.isHard))
+    }
+
+    func testIdentifiersArePreservedExactly() {
+        let violations = PolishOutputValidator.validate(
+            input: "send https://example.com/a to dev@example.com using user_id",
+            output: "send it to the team",
+            dictionary: .empty,
+            lengthRatio: 0.5...2
+        )
+        XCTAssertTrue(violations.contains { violation in
+            if case .missingIdentifiers(let values) = violation {
+                return values.contains("https://example.com/a")
+                    && values.contains("dev@example.com")
+                    && values.contains("user_id")
+            }
+            return false
+        })
+    }
+
+    func testNumbersLengthAndLanguageAreObservationOnly() {
+        let violations = PolishOutputValidator.validate(
+            input: "项目 123 明天下午交付并通知全部相关成员",
+            output: "Ship tomorrow.",
+            dictionary: .empty,
+            lengthRatio: 0.9...1.1
+        )
+        XCTAssertFalse(violations.isEmpty)
+        XCTAssertTrue(violations.filter(\.isHard).isEmpty)
+    }
+}