Testing output doesn't capture the viewport when the terminal content scrolls

### Describe the bug

When a `.tape` file generates enough output to cause the terminal to scroll, the testing/golden file outputs (`.txt`, `.ascii`, `.test`) fail to capture the current viewport. Instead of saving the visible lines, `vhs` repeatedly writes the lines from the top of the buffer.

This issue likely stems from the `VHS.Buffer` method, which appears to [not account for the scroll position](https://github.com/charmbracelet/vhs/blob/1b7db05a951df65e30eb4fdb9429711e89c7e511/testing.go#L70) when generating the output frame.

### Setup

- OS: macOS (15.6.1)
- Shell: `zsh`, `bash`, `nu`

### To Reproduce

1. Create a file named `testing-output.tape` with the following content.

```tape
Set Height 300
Output testing-output.ascii

Type "echo $LINES"
Enter

Type "seq 10"
Enter

Type "echo foo"
Enter
```

2. Run the tape file: `vhs testing-output.tape`.
3. Inspect the generated `testing-output.ascii` file.

### Actual Behavior

The output file captures the top of the buffer at each step. The final frames are missing the scrolled output from `seq 10` and the subsequent `echo foo` command.

```
────────────────────────────────────────────────────────────────────────────────
> echo $LINES

────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
>

────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
> seq 10

────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
> seq 10
1
2
3
────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
> seq 10
1
2
3
────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
> seq 10
1
2
3
────────────────────────────────────────────────────────────────────────────────
```

### Expected Behavior

The output file should accurately reflect the terminal's viewport at each step, showing the content as it scrolls into view, e.g., like this:

```
────────────────────────────────────────────────────────────────────────────────
> echo $LINES

────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
>

────────────────────────────────────────────────────────────────────────────────
> echo $LINES
6
> seq 10

────────────────────────────────────────────────────────────────────────────────
6
7
8
9
10
>
────────────────────────────────────────────────────────────────────────────────
6
7
8
9
10
> echo foo
────────────────────────────────────────────────────────────────────────────────
8
9
10
> echo foo
foo
>
────────────────────────────────────────────────────────────────────────────────
```

Start in testing.go at the linked VHS.Buffer method and reproduce the issue with the provided testing-output.tape steps. Verify that generated .txt, .ascii, and .test outputs reflect the terminal's current viewport after scrolling, including the final seq 10 and echo foo frames.

Written by the indexing model from the issue text.