moonbitlang / core / 4090

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fix(list): abort on negative repeat count

`@list.repeat(n, x)` previously treated negative `n` as `0` and
silently returned `Empty`. Negative `n` is almost always a caller bug
(sign error / off-by-one), so abort with `"negative repeat count"`
instead. An `n` of `0` still returns the empty list.

This rounds out the same treatment applied to `Array::repeat`,
`String::repeat`, `StringView::repeat`, and `Bytes::repeat`.

Refs #3498. Split out from #3505 for easier review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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