Add rc.slice custom operator (#7505)

* Add rc.sortBy custom operator

Orders an array by a key computed per item, ascending, with ties keeping
their input order. Keys must be all strings or all numbers so the two
engines never have to invent a cross-type ordering.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Add rc.slice custom operator

Takes a run of elements out of an array, with the start and length
semantics substr already uses for strings. Pairs with rc.sortBy to
express top-N without a reduce carrying a compound accumulator.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Exempt the custom operator dispatcher from cyclomatic complexity

The tenth operator pushed the switch past the limit. Operators.dispatch
carries the same exemption for the same reason.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Reject non-finite rc.sortBy keys

A NaN key compares false in both directions, so every pair tied and the
input came back unsorted with no error. Negating a string key to attempt
a descending sort produced exactly that.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Note ascending order in the rc.sortBy summary

Co-authored-by: Cursor <cursoragent@cursor.com>

* Trim the rc.sortBy doc comment

Co-authored-by: Cursor <cursoragent@cursor.com>

* Trim the rc.sortBy stability comment

Co-authored-by: Cursor <cursoragent@cursor.com>

* Drop the dispatcher comment above the lint annotation

Co-authored-by: Cursor <cursoragent@cursor.com>

* Scope the substr hint to string input in rc.slice

Co-authored-by: Cursor <cursoragent@cursor.com>

* Pin rc.slice index and source type strictness in fixtures

Co-authored-by: Cursor <cursoragent@cursor.com>

* Reach the float index branch through arithmetic in rc.slice fixtures

Co-authored-by: Cursor <cursoragent@cursor.com>

* Order rc.sortBy string keys by UTF-16 code unit

Swift's < orders by Unicode scalar and treats canonically equivalent
strings as equal, while JS and Kotlin order by UTF-16 code unit. The two
disagree once a scalar above the surrogate range meets an astral one, so
the same rule would sort differently on iOS and Android.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Keep the rc.sortBy ordering fixtures platform-neutral

The fixture files are byte-identical across the iOS and Android engines, so
a description that explains the contract in terms of one host language's
string comparison does not read correctly in the other repository.

Co-authored-by: Cursor <cursoragent@cursor.com>

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Co-authored-by: Cursor <cursoragent@cursor.com>