Uno Game: Simulate an Uno Card Game Round and Determine the Valid Plays for the Current Player

Question

## Problem

Implement the core logic of an Uno game. A play is valid if the card matches the top of the discard pile by color OR by number/action, or if the card is a Wild. Implement:
- `valid_plays(hand: list[str], top_card: str) -> list[str]`
- `apply_play(card: str, state: GameState) -> GameState` -- applies effects (Skip, Reverse, Draw Two, Wild Draw Four).

```python
# Card format: "RD" = Red Draw Two, "B7" = Blue 7, "W" = Wild, "WD4" = Wild Draw Four
# Colors: R=Red, G=Green, B=Blue, Y=Yellow

def valid_plays(hand: list[str], top_card: str) -> list[str]:
    pass
```

**Example:**
```
hand     = ["R3", "B7", "GS", "W", "Y3"]
top_card = "R7"
output   -> ["R3", "B7", "W", "Y3"]
# R3: matches Red; B7: matches 7; W: always valid; Y3: matches 3; GS: no match
```

## Follow-ups
1. A player has only Wild Draw Four cards. The rules state you can only play it if you have no other valid card. How do you enforce this with hidden hands?
2. Implement the full turn loop for a 4-player game, handling direction reversal from Reverse cards.
3. How would you write a simple AI that picks the best card to play (greedy: prefer special cards last)?
4. Represent the game state as a serializable object so the game can be saved and resumed.

Uno Game: Simulate an Uno Card Game Round and Determine the Valid Plays for the Current Player

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