Custom validators for shape props

Add constraints to shape props with the .check() and .refine() validator methods.

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Shape props are validated whenever a shape record is written to the store. The validators from T (re-exported from @tldraw/validate) can be extended: .check(name, fn) adds a step that throws on invalid input without changing the value, and .refine(fn) returns a new value, so it can transform as well as validate.

The example shape has two constrained props:

• percentage chains two .check() calls so values below 0 or above 100 throw. The check name appears in the error message.
• rating uses .refine() to clamp values into the 1-5 range instead of rejecting them.

On load, the example creates a valid shape, then tries to create one with percentage: 150 (which throws; see the browser console), then creates one with rating: 10 (which succeeds and is stored as 5).