markdown-it linkify: true has two quadratic paths, so a few hundred KB of markdown blocks the event loop for tens of seconds

Summary

## 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 take 42ms. So it needs many children AND many of them linkifying. For the second, replaying just the `SCHEME_RE` calls against the same growing prefixes with no markdown-it involved accounts for 11.7s of the 15s.

Ordinary prose does it too. `'ping a@b.co ok\n'.repeat(20000)` is 293KB and takes 31s.

## Caveat

`linkify` is off by default, so this only reaches apps that turn it on.

## Impact

Availability only. A few hundred KB of fairly ordinary markdown pins one core for tens of seconds, and because it's quadratic it degrades quickly with size. Nothing is read, written or executed.