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perf(builtin): make Array::swap inlineable

Follow-up to #3716. `Array::swap` is a small, hot accessor (used throughout
sorting / partitioning / shuffling) that was left out of #3716 because its
abort message carries two indices and could not reuse the single-index
`index_out_of_bounds` helper.

Like the accessors fixed in #3716, its inline interpolated
`abort("... index is (\{i}, \{j})")` builds a `StringBuilder` and inflates the
body past clang's inline-cost threshold (cost=995 vs 225). The backend emits it
as an out-of-line call, so loops that call `swap` cannot inline it.

Add a two-index `index_out_of_bounds2` cold helper (`#inline(never)`) and route
`Array::swap` through it. The body drops to cost ~70 and clang inlines `swap`
into its callers; the exact diagnostic message is preserved.

Validation:
- moon check --target native (clean)
- moon test --target native -p builtin (2835 passed)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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