Add some basic Claude code rules (#1065)

CLAUDE.md

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+- Always prefer best practices when writing code.
+- Write using concise, legible, and easily maintainable code.
+- Avoid repetition where possible, but not at the expense of code legibility.
+- Type assertions are an absolute last resort. In almost all cases, they are a crutch that leads to brittle code.
+- This codebase has extensive eslint autofix rules and IDEs are configured to use eslint as the format on save tool. Run `eslint --fix` instead of manually figuring out whitespace fixes or other trivial style concerns. Review the results and correct any remaining eslint errors.
+- Take advantage of `TypedArray` `subarray` when appropriate.
+- The `size` and `pos` properties of `Rectangle` share the same array buffer (`subarray`); they may be used to set the rectangles size and position.
+- Prefer single line `if` syntax over adding curly braces, when the statement has a very concise expression and concise, single line statement.
+- Do not replace `&&=` or `||=` with `=` when there is no reason to do so. If you do find a reason to remove either `&&=` or `||=`, leave a comment explaining why the removal occurred.
+- You are allowed to research code on https://developer.mozilla.org/ and https://stackoverflow.com without asking.
+- When adding featueres, always write vitest unit tests using cursor rules in @.cursor
+- When writing methods, prefer returning idiomatic JavaScript `undefined` over `null`, unless there is a specific use-case for `null`.