perf(js): check String.ends_with without reversing the inputs

[oxura]

Fixes #1641.

JS no longer builds two reversed strings for a suffix check. The new intrinsic uses native endsWith, then checks that a matching suffix starts at a Bend Char boundary when U+DC00 occurs. The C implementation and checked Base definition stay unchanged.

A local exported-module probe uses 50,001 characters (ASCII plus an emoji):

Case	Before	After, warmed
Present suffix	5–7 ms	0.056–0.063 ms
Missing suffix	5–9 ms	0.053–0.081 ms
Empty suffix	5–9 ms	0.054–0.125 ms

Bun 1.3.13/Linux x86-64; whole API calls including host string handling, not a cross-platform benchmark. Boundary regressions cover ordinary text, empty and whole patterns, astral characters, packed payloads and a false UTF-16 match beginning inside an astral pair. AI-assisted.

[oxura]

This is JS-only performance work, so I’m withdrawing it following the reference-runtime policy in #1644.