Pin UTF-16 string comparison in the predicate fixtures (#4106)

### Checklist
- [x] If applicable, unit tests
- [ ] If applicable, create follow-up issues for `purchases-ios` and
hybrids

### Motivation
Port of RevenueCat/purchases-ios#7518. The iOS engine compared strings
with Swift's own operators, which order by Unicode scalar and read
canonically equivalent spellings as equal, so relational operators,
equality, `in` and `rc.entries` key ordering could evaluate differently
on the two platforms. Kotlin already compares UTF-16 code units, so no
behavior changes here.

### Description
- 15 fixtures pinning code-unit semantics for `<`, `<=`, `>`, `>=`,
`==`, `===`, `in` and `rc.entries` key order. All pass against the
engine as it stands.
- Fixture files stay byte-identical with `purchases-ios`;
`rc_length.json` loses a description that explained the contract in
Swift's terms, which never read correctly here.
- The `rc.entries` doc now names UTF-16 code unit order, following the
iOS wording.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Test and documentation updates only; predicate evaluation logic is
unchanged on Android/Kotlin.
> 
> **Overview**
> Adds **15 shared predicate conformance fixtures** (fixture count **519
→ 534**) that lock in **JavaScript-style UTF-16 code unit** behavior for
string compares, equality, `in`, and `rc.entries` key ordering—aligned
with the iOS port so cross-platform rules do not diverge on surrogate
pairs or canonically equivalent spellings (e.g. precomposed vs
decomposed é).
> 
> **Docs-only** change: `rc.entries` KDoc now says object keys sort in
**UTF-16 code unit order** instead of generic “lexicographic.”
`rc_length.json` fixture copy is updated to describe the emoji length
case in JS/Kotlin terms rather than Swift grapheme wording.
> 
> **No Kotlin engine behavior change** in this PR; new fixtures pass
against the existing implementation.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
3cdbb924983f2bca146d56f2413b2c94018fc2c1. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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