Add default camera listing and selection

[seanfight]

Describe your change:

Add computer_vision/default_camera.py so a computer’s cameras can be listed, a preferred camera can be saved, and (optionally) an OS-level default can be applied.

Windows and Linux do not expose a single global “default camera” API. Apps typically open the first available capture device. This utility:

1. Discovers cameras (Windows PnP, Linux V4L2, macOS system_profiler, OpenCV fallback)
2. Selects one by index, name fragment, or device path
3. Saves the choice to a JSON preference file
4. With --system, applies an OS-level default:
   • Windows: enable the chosen camera and disable the others (run as Administrator)
   • Linux: write a udev rule and ~/.config/video-default symlink
   • macOS: preference only (no system-wide API)

Usage:

python computer_vision/default_camera.py list
python computer_vision/default_camera.py set 1
python computer_vision/default_camera.py set "usb" --system
python computer_vision/default_camera.py get

使用方法（中文）：

python computer_vision/default_camera.py list          # 列出摄像头
python computer_vision/default_camera.py set 1         # 把 1 号设为默认
python computer_vision/default_camera.py set "usb" --system  # 设为系统默认（Windows 需管理员）
python computer_vision/default_camera.py get           # 查看当前默认

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.