fix(remend): drop lookbehind from single-tilde pattern for Safari < 16.3

[s-andringa]

Refs #519.

What

Removes the lookbehind assertion from remend's single-tilde escape pattern:

-const SINGLE_TILDE_PATTERN = /(?<=[\p{L}\p{N}_])~(?!~)(?=[\p{L}\p{N}_])/gu;
+const SINGLE_TILDE_PATTERN = /([\p{L}\p{N}_])~(?!~)(?=[\p{L}\p{N}_])/gu;

The preceding word character is now captured and written back in the replacement, and the offset handed to isInsideCodeBlock is shifted by its length so it still points at the ~. Semantics are unchanged — Unicode support (\p{L}, \p{N}, so CJK and other scripts) is fully preserved.

Why

JSCore on iOS 16.0–16.2 / Safari < 16.3 does not support lookbehind and throws SyntaxError: Invalid regular expression: invalid group specifier name. Because the pattern is a module-level constant, this throws during module evaluation — before any user code runs — so the whole chunk fails to export and the page goes blank. There is no way for a consumer to catch it. Safari only shipped lookbehind in 16.4 (caniuse).

Scope

This PR deliberately only fixes remend. #519 identifies a second lookbehind in mdast-util-gfm-autolink-literal@2, pulled in via remark-gfm@4. That one is not addressed here, because the maintainer has stated the lookbehind is intentional and will not be changed: syntax-tree/mdast-util-gfm-autolink-literal#10 (comment). Projects needing full iOS 16.0–16.2 support will have to patch that dependency themselves (pnpm patch / patch-package).

So this change does not single-handedly close #519 for every streamdown consumer. It does fix it outright for anyone depending on remend directly, without dragging in remark-gfm — which is my own use case, and the one bit of this that is squarely this repo's own code rather than a third party's.

Testing

• pnpm --filter remend test — 458/458 pass, no changes needed to existing tests (behaviour is identical)
• pnpm --filter streamdown test — 991/991 pass
• pnpm check — clean
• Built dist output verified free of any (?< occurrences

Changeset included (remend, patch).

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