# Role

You are a Senior Flutter Developer.
You are continuing the development of "Colosseum's Choice".
The basic battle prototype is already working.
Now, you need to implement the **Item & Progression System**.

# Goal

Modify the existing code to implement the following features:

1.  **Item Model:** Create items that boost stats (ATK, MaxHP).
2.  **Inventory System:** Player can equip items, and stats are calculated dynamically (Base + Item Bonus).
3.  **Battle Loop:**
    - **Victory:** When Enemy HP <= 0, show a dialog to choose 1 of 3 random items.
    - **Progression:** After picking an item, the next battle starts immediately with a slightly stronger enemy.
    - **HP Rule:** Player HP is NOT fully restored between battles (Roguelike element).

# Required Changes & Implementation Details

Please generate the updated code for the following files.
**IMPORTANT:** Preserve the existing "Risk vs Return" and "Armor Decay" logic.

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## 1. `lib/models/item.dart` (New File)

**Description:**

- **Fields:** `String name`, `String description`, `int atkBonus`, `int hpBonus`.
- **Constructor:** Standard constructor.

## 2. `lib/models/character.dart` (Modify)

**Description:** Update to support equipment.

- **New Fields:** `List<Item> equipment`.
- **Stat Logic:**
  - `int get totalAtk`: Returns `baseAtk` + sum of all equipped items' `atkBonus`.
  - `int get totalMaxHp`: Returns `baseMaxHp` + sum of all equipped items' `hpBonus`.
  - **Important:** Use `totalAtk` and `totalMaxHp` for battle logic instead of raw fields.
- **Methods:**
  - `void equip(Item item)`: Add to equipment. If `hpBonus` > 0, increase current `hp` by that amount as well (optional heal).

## 3. `lib/providers/battle_provider.dart` (Modify)

**Description:** Handle Victory and Stage Progression.

- **New Properties:**
  - `int stage`: Tracks current stage number (starts at 1).
  - `List<Item> rewardOptions`: Stores the 3 random items generated upon victory.
  - `bool showRewardPopup`: Flag to trigger UI dialog.
- **Methods:**
  - `initializeBattle()`: Reset Player (Stage 1).
  - `_onVictory()` (Internal): Called when Enemy dies. Generate 3 random items (e.g., "Rusty Sword (+2 ATK)", "Leather Vest (+10 HP)"). Set `showRewardPopup = true`.
  - `selectReward(Item item)`: Equip item to player -> Increase Stage -> Spawn stronger Enemy (Scale Enemy stats by Stage) -> Reset `showRewardPopup`.
  - **Update `playerAction`**: Ensure it uses `player.totalAtk` for damage calculation.

## 4. `lib/screens/battle_screen.dart` (Modify)

**Description:** Add UI for stats and rewards.

- **Top Area:** Display `Stage: X`. Update HP bars to show `current / totalMaxHp`.
- **Victory Handling:**
  - Use `Consumer` to listen to `battleProvider`.
  - If `provider.showRewardPopup` is true, show a `SimpleDialog` (or similar) listing the `rewardOptions`.
  - Clicking an option calls `provider.selectReward(item)`.

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# Output Format

Please provide the complete code for the modified/new files.