feat: 独立词库、三向认词与当日快闪复习

## 独立词库

理解词原先只能跟着课程单元走，学完 A0 十课词汇量只增加约 22 个实词，
不足以解决"记不住单词"。新增一份独立词库 assets/words/wordbank.json
（2748 词，A1–B1），挂进 receptiveWordRegistry 的合成单元 bank-A1/A2/B1，
完全复用理解词已有的状态机，不依赖课程进度，第一天就能用。

数据来源、许可与合成规则记在 tool/words/DATA-NOTE.md：CEFR-J 定等级、
公开词书提供音标、AI 重写全部释义并生成例句、OpenSubtitles 提供口语词频。
词书部分为 CC BY-NC-SA 4.0 且上游权利不明，仅供个人非商用；
若要分发或上架，须替换音标那一列。

## 背单词机制

- 间隔阶梯 1/3/7/15/30/60/120 天，连续答对上一级，答错回第一级。
  原先首次答对后要等 7 天才复习，正是"第二天就忘"的成因。
- 每日新词上限（10 分钟 8 个 / 20 分钟 15 个 / 30 分钟 20 个）。
  阶梯第一级是次日，今天引入的新词就是明天的工作量。
- 新词按口语频率发放，不再按字母序 —— A1 从 a.m./ability 变成 no/not/know/just。
- 三个方向按层级轮转：看词（英→中）→ 听词（音→中）→ 想词（中→英）。
  想词题仍是选择题，不要求产出，理解词定位不变，不进升级分母。
- 单词页独立成 tab，首页今日任务卡下方给一张认词入口卡。

## 用法对照

课程 JSON 增加 usage 字段（when/reply/swap/confuse）：一个句型用在什么场合、
对方通常怎么答、还能怎么说、跟哪个学过的句型容易混。
知道 How are you? 的意思，不等于知道它不是用来问名字的。

## 复习流

- 当日快闪（recap）独立成队列，不占复习预算，也不计入积压。
- 只发放当日预算内的量，其余保持到期状态等下次，不悄悄丢弃或改期。
- 答错的项隔几题后回来，而不是立刻重问。

测试 296 通过，flutter analyze 干净。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kouyu_english/lib/core/models.dart

@@ -2,7 +2,7 @@ enum LearningGoal { dailyLife, travel, workStarter }
 
 enum PlacementLevel { beginner, someBasics, simpleConversation }
 
-enum AppTab { learn, dialogue, review, profile }
+enum AppTab { learn, dialogue, review, words, profile }
 
 enum LessonStep {
   preview,
@@ -18,6 +18,72 @@ enum LessonStep {
 
 enum MasteryStatus { newItem, recognize, recall, use, master, needsReview }
 
+/// Learning engine 3.5: a recognition-only word has its own three states,
+/// deliberately shorter than [MasteryStatus]. It is never asked to be spelled
+/// or said, and it never counts toward a level's upgrade denominator.
+enum WordStatus { newWord, recognized, familiar }
+
+/// What the app knows about one recognition-only word.
+class WordKnowledge {
+  const WordKnowledge({
+    required this.id,
+    required this.status,
+    this.recognizedAt,
+    this.dueAt,
+    this.misses = 0,
+    this.step = 0,
+    this.startedAt,
+  });
+
+  final String id;
+  final WordStatus status;
+
+  /// When it first became 「认识」. 「熟悉」 needs a second success at least
+  /// seven days after this, on a different question.
+  final DateTime? recognizedAt;
+
+  /// When it comes up for a check again; null once nothing is scheduled.
+  final DateTime? dueAt;
+
+  /// How often recognition has failed, used to surface the weak ones.
+  final int misses;
+
+  /// Successes in a row, which is the rung of the spacing ladder this word is
+  /// on. A miss puts it back to 0.
+  final int step;
+
+  /// When the word entered the learner's list, which is what the daily cap on
+  /// new words is counted against.
+  final DateTime? startedAt;
+
+  WordKnowledge copyWith({
+    WordStatus? status,
+    DateTime? recognizedAt,
+    DateTime? dueAt,
+    int? misses,
+    int? step,
+    DateTime? startedAt,
+  }) => WordKnowledge(
+    id: id,
+    status: status ?? this.status,
+    recognizedAt: recognizedAt ?? this.recognizedAt,
+    dueAt: dueAt ?? this.dueAt,
+    misses: misses ?? this.misses,
+    step: step ?? this.step,
+    startedAt: startedAt ?? this.startedAt,
+  );
+
+  Map<String, dynamic> toJson() => {
+    'id': id,
+    'status': status.name,
+    'recognizedAt': recognizedAt?.toIso8601String(),
+    'dueAt': dueAt?.toIso8601String(),
+    'misses': misses,
+    'step': step,
+    'startedAt': startedAt?.toIso8601String(),
+  };
+}
+
 enum EvidenceKind {
   exposure,
   assisted,
@@ -392,6 +458,13 @@ class SentenceAnalysisResult {
       );
 }
 
+/// What a queued review is for. A `checkpoint` task carries the four spaced
+/// checkpoints of learning engine 3.1. A `recap` task is the same-day recall
+/// after a segment, or the re-ask of an item missed earlier in the session:
+/// it only strengthens encoding, so it never advances a checkpoint and a miss
+/// is never counted as a language failure.
+enum ReviewKind { checkpoint, recap }
+
 class ReviewItem {
   const ReviewItem({
     required this.id,
@@ -400,6 +473,7 @@ class ReviewItem {
     required this.hint,
     required this.dueAt,
     required this.skill,
+    this.kind = ReviewKind.checkpoint,
     this.attempts = 0,
     this.successfulReviews = 0,
     this.variantIndex = 0,
@@ -413,6 +487,7 @@ class ReviewItem {
   final String hint;
   final DateTime dueAt;
   final String skill;
+  final ReviewKind kind;
   final int attempts;
   final int successfulReviews;
   final int variantIndex;
@@ -421,6 +496,7 @@ class ReviewItem {
 
   ReviewItem copyWith({
     DateTime? dueAt,
+    ReviewKind? kind,
     int? attempts,
     int? successfulReviews,
     int? variantIndex,
@@ -436,6 +512,7 @@ class ReviewItem {
     hint: hint ?? this.hint,
     dueAt: dueAt ?? this.dueAt,
     skill: skill ?? this.skill,
+    kind: kind ?? this.kind,
     attempts: attempts ?? this.attempts,
     successfulReviews: successfulReviews ?? this.successfulReviews,
     variantIndex: variantIndex ?? this.variantIndex,