wgsl: Change the term bounds to endpoints for intervals

[zoddicus]

This is to help differentiate between concept of the edges of an interval and the values that define a numeric space, like f32 precision, which previously have both been called bounds.

Issue #3201

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[jiangzhaoming]

Looks good to me with some minor typo in comments, thanks!

[jiangzhaoming]

Suggested change

	// because sqrt(1 - x*x) = sqrt(0), and sqrt is defined in terms of
	// because sqrt(1 - x*x) = sqrt(0), and sqrt is defined in terms of