Create and fix Unity 2D physics

Do not weigh 3D physics options. If evidence shows the user means 3D, say so and stop.

Prefer WebFetch over WebSearch — faster and lands on the exact reference. Only fetch what you need. Replace <VERSION> with the following before fetching:

• docs.unity.com: The Unity version, e.g. 6000.7. Pages are markdown — URLs end in .md.
• Package on docs.unity3d.com: The package version, e.g. @1.1. Check Packages/manifest.json if unsure.

Important

• Use the unity cli (unity command --format json) and the unity-cli and unity-pipeline skills for each step.
• Don't use using directives or unqualified types in the snippet for eval. The compiler reads using UnityEngine and a bare AssetDatabase, Volume, or Object causes errors (CS0246 / CS0103 / CS0104). return what you want to read; it arrives at data.result.result.
• Create a restore point you can roll back to if your changes fail.
• Do only what's asked. Don't change unrelated assets or files.
• Avoid long explanations.
• Never leave verification code in the user's script; never build logging or settle-detection they didn't ask for. See Final step.
• Never reflect over types to discover an API — fetch the member page instead. The system probe is the one exception.

Step 0: Which physics system