feat(numpy): support negative indices in checked array accessors (#6201)

* reject negative indices in numpy array bounds checks

* fix(numpy): clarify negative index errors and document no wrapping

Use a single unsigned bounds compare, say in the error message that
negative indices are not supported, document the behavior on the
accessors, and cover offset_at/offset_at_t in test_bounds_check.

Assisted-by: ClaudeCode:claude-opus-5-5

* feat(numpy): support negative indices in checked array accessors

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Co-authored-by: sayed ahrar <sayed@sproutxp.com>
Co-authored-by: Henry Schreiner <henryfs@princeton.edu>