fix(polish): gate test-build style generation at 1,250 chars and cap export at 5,000

Remove the test-mode unlimited personal-style corpus bypass and replace it with a real 1,250-character minimum gate. Production stays at 2,500. Training-corpus export and the diagnostics view now use a 5,000-character upper bound so the user's full private history never leaves the device in a single export.

- Add PolishStyleLearningCorpusBuilder.testBuildEffectiveCharacterCount (1,250) and trainingExtractionMaximumCharacterCount (5,000); trainingWindow(from:maximumCharacterCount:) accepts a custom cap.
- PolishStylesView: drop bypassesStyleLearningCharacterGate in favor of styleLearningMinimumCharacterCount, unify the ready/remaining text, remove the obsolete polishStyles.learn.testBuildReady key.
- PolishStyleCorpusExportStore: pass the 5,000 cap to trainingWindow and expose trainingExtractionMaximumCharacterCount in the export schema.
- FlowDiagnosticsSettingsView: show the export-sized (5,000-cap) corpus so the diagnostics match what the export ZIP actually contains.
- Tests: rewrite the export cap test for 5,000, add below-cap coverage, add trainingWindow(maximumCharacterCount:) unit coverage, and convert the old UI test into a regression test for the 1,250 test-build minimum.
- L10n: drop the testBuildReady string in en + zh-Hans, update diagnostics.corpus.ready to mention 5,000.
- CHANGELOG: bilingual entry under [Unreleased].

OSGKeyboardTests/PolishStyleLearningServiceTests.swift

@@ -146,6 +146,42 @@ final class PolishStyleLearningServiceTests: XCTestCase {
         XCTAssertEqual(window.examples.map(\.finalText), ["older", "newer"])
     }
 
+    func testTrainingWindowHonorsCustomMaximumCharacterCount() {
+        // Total available characters: 7,000. With a 5,000-character
+        // cap (training-corpus export), newest (3,000) + middle (2,000)
+        // fills the window and oldest is dropped.
+        let oldest = PolishStyleLearningExample(
+            prePolishText: String(repeating: "旧", count: 2_000),
+            finalText: "oldest",
+            polishStyleID: nil,
+            createdAt: Date(timeIntervalSince1970: 1)
+        )
+        let middle = PolishStyleLearningExample(
+            prePolishText: String(repeating: "中", count: 2_000),
+            finalText: "middle",
+            polishStyleID: nil,
+            createdAt: Date(timeIntervalSince1970: 2)
+        )
+        let newest = PolishStyleLearningExample(
+            prePolishText: String(repeating: "新", count: 3_000),
+            finalText: "newest",
+            polishStyleID: nil,
+            createdAt: Date(timeIntervalSince1970: 3)
+        )
+
+        let window = PolishStyleLearningCorpusBuilder.trainingWindow(
+            from: [oldest, newest, middle],
+            maximumCharacterCount: PolishStyleLearningCorpusBuilder
+                .trainingExtractionMaximumCharacterCount
+        )
+
+        XCTAssertEqual(window.effectiveCharacterCount, 5_000)
+        XCTAssertEqual(
+            window.examples.map(\.finalText),
+            ["middle", "newest"]
+        )
+    }
+
     func testGenerationRunsExtractorBeforeSynthesizerWithSeparatedPayloads() async throws {
         var catalog = PolishStyleCatalog()
         let activeStyle = PolishStylePack(