Feature request: Add methods to control audio playback (pause/stop) for add_sound

Nobody has claimed this yet.

## Description of proposed feature

Currently, `Scene.add_sound()` allows users to add background music or sound effects to animations. However, once a sound is added via `add_sound()`, there is **no way to stop or pause it** during the animation. The audio plays in its entirety and **determines the final video length**—the rendered video will continue running until the audio file finishes, regardless of when the animation actually ends.

This creates several problems:
- For short animations, long background music unnecessarily extends the video, forcing users to wait or manually edit the audio file.
- For dynamic animations where music should stop at a specific moment (e.g., a dramatic reveal or a punchline), there is no way to achieve this within Manim.
- Users are forced to either pre-cut their audio files (which is tedious and breaks when animation lengths change) or use external video editors (which adds an extra step to the workflow).

I propose adding a `stop_sound()` method (and potentially `pause_sound()` / `resume_sound()`) to the `Scene` class. When called, `stop_sound()` would immediately halt all currently playing audio tracks added via `add_sound()`. Additionally, the **video duration should be determined solely by the animation timeline**, not by the length of the audio file. If the animation ends before the audio finishes, the video should stop, and the remaining audio should be truncated.

## How can the new feature be used?

```python
from manim import *

class ExampleScene(Scene):
def construct(self):
# Start background music
self.add_sound("background.mp3")

# Some animations
circle = Circle()
self.play(Create(circle), run_time=2)
self.wait(1)

# Stop the music right before a dramatic moment
self.stop_sound()

# The final animation plays silently (or with a short sound effect)
square = Square()
self.play(Transform(circle, square), run_time=1)
```

This would be particularly useful for:
- **Educational content**: Stop music when the key explanation begins.
- **Storytelling animations**: Use audio cues to match visual beats.
- **Short-form content**: Ensure videos end exactly when the animation ends, without silent trailing or unnecessary extension.

## Additional comments

This issue has been a known pain point in the Manim community for years. A [Stack Overflow question from 2020](https://stackoverflow.com/questions/64236768/how-to-pause-or-remove-sounds-in-manim) asked about stopping audio, and the answer was simply that "Manim doesn't support pausing or stopping sound files." Since then, the community has had to rely on workarounds:
- Pre-trimming audio files (requires knowing exact animation length upfront).
- Rendering without audio and adding music in external editors (breaks the "render once" workflow).

I've also noticed that the `manim-voiceover` plugin has more advanced audio handling capabilities, suggesting that this functionality is technically feasible. A native implementation in the core library would greatly improve the user experience.

**Additional context**: I'm aware that Manim is currently undergoing a major refactor and new features may not be prioritized. However, I believe this feature addresses a fundamental UX issue and could be considered for inclusion after the refactor is complete, or as part of it if the audio system is being redesigned.

Start by tracing Scene.add_sound and the current audio-rendering flow; the issue does not name implementation files or tests. Compare the audio handling in the manim-voiceover plugin as context for possible pause, resume, and stop behavior. Done should include controllable playback and video duration based on the animation timeline, with remaining audio truncated when the animation ends.

Written by the indexing model from the issue text.