Add At and AtKV index-based lookup functions #minor

[freeformz]

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[Copilot]

Pull request overview

This PR adds two new index-based lookup functions (At and AtKV) to the seq library, following the established dual API pattern. These functions allow retrieving values by their 0-based position in a sequence, complementing the existing Find family of functions that search by value or predicate.

Changes:

• Added At and AtKV functions for index-based lookup in sequences with proper error handling for negative and out-of-range indices
• Added comprehensive example tests covering success, out-of-range, empty, and negative index cases
• Updated README documentation to list the new functions under the "Finding" section

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
seq.go	Implements At and AtKV functions with proper handling of edge cases (negative indices, out of range)
seq_test.go	Adds example tests for both functions covering various scenarios
README.md	Documents the new functions in the Search/Finding section
CLAUDE.md	Adds documentation note about Go version changes requiring #minor tag (minor documentation improvement)

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