feat(vec): add extend() method for batch element insertion

[gabrielrondon]

Summary

• Add BaseVec::extend(&[T]) and Vec::extend(&[T]) for appending multiple items in a single operation
• Writes all items sequentially and updates the length once, reducing stable memory writes for the length field compared to repeated push()
• Follows the same pattern as push(): serialize each item, write entry size, write data, then update length at the end
• Includes unit tests for empty slice, basic usage, extend after push, multiple extends, and persistence across init/reinit

Closes #349

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