[oxura]

Fixes #1637.

Keep #1628's boundary walk when a haystack contains U+DC00. Otherwise native substring search is safe: there cannot be a match inside a packed U32 Char. An astral pair containing U+DC00 conservatively keeps the existing path.

Measured through an actual exported Bend function on Bun 1.3.13/Linux, rejecting a 1024-character almost-match in one million ASCII characters:

Call	Before	After
String.contains	77–79 ms	0.30–0.37 ms

Added consumer regressions for ordinary text, empty patterns, astral characters, a match inside a packed payload, a false UTF-16 match across an astral boundary, and a genuine match after a packed Char. Generated C and JS print all seven expected answers. Repository gate: 54/54. Mini-cluster/Metal/CUDA not run. AI-assisted.

[Lorenzobattistela]

Thanks for the careful work and the measurements. We're closing this one: the JS backend is Bend's reference runtime, and we don't want ad-hoc performance special cases there. Performance work belongs on the C side. #1628's boundary walk is the behavior we want to keep in JS.

Separately, guide/GUIDE.md is written by hand. Please leave it out of PRs, including rewording to make room for new lines.

[oxura]

Got it — I’ll leave GUIDE.md alone and keep performance changes on the C side. Thanks for spelling that out.