Add rc.slice custom operator

[ajpallares]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

Keeping a run of an array has no operator: it composes from rc.entries + filter + map, but the index arrives as a string and negative-index clamping is hand-written, so simple filters like "the last three events" can be really hard to express and read.

Description

{"rc.slice": [array, start]} or {"rc.slice": [array, start, length]}.

• Start and length mean what they already mean in substr: negative start counts from the end, negative length drops from the right, and both clamp instead of failing. The third argument is therefore a count, not an end index — deliberately substr's convention rather than JS slice's, so the engine has one indexing model.
• Arrays only. A string throws and the message points at substr.
• Indices must be whole numbers. .float is accepted since all arithmetic returns it, so length - 1 works; fractional or non-numeric throws instead of being truncated or coerced as substr does.

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Note

Low Risk
Isolated new Rules Engine operator with broad fixture coverage; no changes to existing operator behavior beyond the dispatch switch.

Overview
Adds rc.slice so JSON Logic rules can take a contiguous run from an array without composing rc.entries + filter/map workarounds.

{"rc.slice": [array, start]} or with an optional length third argument. Indexing follows existing substr rules: negative start is from the end, negative length trims from the right, out-of-range values clamp instead of erroring, and the third arg is a count (not a JS-style end index). Only arrays are accepted—strings still use substr, with a hint in the error. Start/length must be whole numbers; whole-valued floats from arithmetic are allowed, but fractional or coerced values throw typeMismatch (stricter than substr).

Wired through CustomOperators.dispatch, added to the Xcode target, and covered by rc_slice.json predicate fixtures (fixture total 544 → 578).

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