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refactor(builtin): declarative reverse-range loop in Array/ArrayView::rev_foldi

Sibling of #3519 (FixedArray::rev_foldi). Same shape transform — replace
`for i = len-1, acc = init; i >= 0; { continue i-1, ... }` with
`for i in len>..0; acc = init { continue ... }` — applied to both
`Array::rev_foldi` and `ArrayView::rev_foldi`.

Hot-path-cautious bucket: same loop, same accumulator, same body — only
the loop header changes. Compiles to the same shape as a c-style
countdown.

Semantics-preserving: `moon test -p builtin` (2825/2825) passes and
`moon info` produces no `.mbti` change.

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

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