Affected: npm: markdown-it

## Summary

Two independent quadratic paths in the `linkify: true` handling. Both are in markdown-it's own code rather than in linkify-it, which stays linear on both payloads.

`src/rules_core/linkify.ts` calls `arrayReplaceAt` once per linkified text token, and that rebuilds the whole `children` array each time. A paragraph of N soft-broken lines is one inline token with about 2N children, so you get N rebuilds over a 2N array. Schema-less emails are what reach it. A `http://` link gets consumed by the inline rule first and never arrives as a text token, so those stay linear.

`src/rules_inline/linkify.ts` runs `state.pending.match(SCHEME_RE)` at every `://` in the source. `state.pending` only gets truncated once a link is actually produced, so an unregistered scheme leaves it growing and every `://` rescans the lot.

## Proof of concept

Clean install of 15.0.0 from npm, `new MarkdownIt({ linkify: true }).render(payload)`, Node 24.

| N | 5,000 | 10,000 | 20,000 | 40,000 | |---|---|---|---|---| | `'a@b.co\n'.repeat(N)`, 34KB to 273KB | 1.1s | 4.7s | 19.6s | 89s | | `'a://'.repeat(N)`, 20KB to 156KB | 0.25s | 0.76s | 3.1s | 15.0s |

Doubling the input roughly quadruples the time in both. With `linkify: false` the same inputs run in 25 to 131ms and stay flat.

Controls for the first one, all at N=20000: putting every email in a single text token (`'a@b.co '`) takes 0.59s, one email per paragraph (`'a@b.co\n\n'`) takes 0.64s, and soft-broken lines with nothing linkifiable tak…