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].

OSGKeyboardShared/Services/PolishStyleLearningService.swift

@@ -150,8 +150,24 @@ public struct PolishStyleLearningCorpus: Equatable, Sendable {
 }
 
 public enum PolishStyleLearningCorpusBuilder {
+    /// Production minimum effective character count required to unlock
+    /// personal style generation. App Store builds enforce this gate.
     public static let requiredEffectiveCharacterCount = 2_500
 
+    /// Test builds (Debug + TestFlight) lower the unlock threshold so
+    /// internal testers can exercise the complete generation pipeline
+    /// without dictating a full production corpus. The threshold is
+    /// still a real gate; the previous "0 / unlimited" bypass is gone.
+    public static let testBuildEffectiveCharacterCount = 1_250
+
+    /// Upper bound on effective characters included in a single
+    /// training-corpus export. Even when the user has accumulated
+    /// significantly more history than the unlock threshold, the
+    /// exported training set never exceeds this cap so the user's
+    /// full private dictation history does not leave the device in
+    /// one shot.
+    public static let trainingExtractionMaximumCharacterCount = 5_000
+
     public static func build(
         from entries: [SpeechHistoryEntry]
     ) -> PolishStyleLearningCorpus {
@@ -167,12 +183,23 @@ public enum PolishStyleLearningCorpusBuilder {
         )
     }
 
-    /// Selects the newest complete examples until the learning threshold is
-    /// reached. If less history is available, every eligible example is kept.
-    /// The returned order is chronological for export and model input.
+    /// Selects the newest complete examples until `maximumCharacterCount`
+    /// is reached. If less history is available, every eligible example
+    /// is kept. The returned order is chronological for export and
+    /// model input.
+    ///
+    /// - Parameter maximumCharacterCount: Hard upper bound for the
+    ///   selected window. Defaults to the production unlock threshold
+    ///   (2,500) so live generation still fits the LLM request budget.
+    ///   Callers that build a training-corpus export should pass
+    ///   `trainingExtractionMaximumCharacterCount` (5,000) so the
+    ///   export can carry up to the broader extraction cap when the
+    ///   user has accumulated more history than the live gate.
     public static func trainingWindow(
-        from examples: [PolishStyleLearningExample]
+        from examples: [PolishStyleLearningExample],
+        maximumCharacterCount: Int = requiredEffectiveCharacterCount
     ) -> PolishStyleLearningCorpus {
+        let limit = max(0, maximumCharacterCount)
         let newestFirst = examples.sorted { $0.createdAt > $1.createdAt }
         var selected: [PolishStyleLearningExample] = []
         var effectiveCharacterCount = 0
@@ -182,7 +209,7 @@ public enum PolishStyleLearningCorpusBuilder {
             effectiveCharacterCount += self.effectiveCharacterCount(
                 in: example.prePolishText
             )
-            if effectiveCharacterCount >= requiredEffectiveCharacterCount {
+            if effectiveCharacterCount >= limit {
                 break
             }
         }