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].

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Monthly usage calendar**: replace the seven-day Home and Mac Dashboard chart with a full current-month calendar whose date-circle opacity reflects each day's dictation characters, placing the Home calendar between its four metrics and Personal Dictionary. / **月度用量日历**：以完整的当月日历替换首页与 Mac 总览的近七天图表，并通过日期圆形背景透明度表示每天的听写字数；首页日历位于四项统计指标与个性词库之间。
 - **Home credits and navigation**: move the credit and invitation card from Settings to Home above the usage metrics, explain free signup and referral rewards before sign-in, and open History or Personal Dictionary directly from their metric cards. / **首页积分与导航**：将积分与邀请卡片从设置迁移到首页统计指标上方，登录前明确说明免费注册与邀请奖励，并支持从听写字数和词库指标卡片直接进入历史记录或个性词库。
 - **Personal style threshold**: allow generating a learned speaking style after 2,500 effective dictation characters instead of 5,000. / **专属风格门槛**：生成学习型说话风格所需的有效听写字符由 5,000 降至 2,500。
+- **Personal style corpus gate**: replace the test-mode unlimited gate with a 1,250-character minimum while keeping the production 2,500-character gate, and cap the training-corpus export at 5,000 effective characters so the user's full private history never leaves the device in one export. / **个人风格语料门槛**：移除测试版本的「无下限」逻辑，改为 1,250 字符最低门槛，正式版保持 2,500 字符；训练语料导出上限调整为 5,000 字符，避免一次性带出全部听写历史。
 
 ### Fixed
 - **Personal style generation**: derive every reviewed prompt through two-stage corpus evidence, apply concrete low-confidence ASR tendencies instead of replacing them with a neutral template, recover wrapped model JSON once, keep one provider configuration and a 45-second budget across both stages, and distinguish cancellation from timeout. / **专属风格生成**：每个待审阅 Prompt 均通过两阶段语料证据生成，并在证据较少时仍应用原始 ASR 中具体的低置信度表达倾向，而非替换为中性模板；同时支持一次模型 JSON 包装恢复，在两阶段固定同一服务配置与 45 秒预算，并区分主动取消和请求超时。