smol-toml: Quadratic-time parse() from parseKey rescanning to end of document on each key line

Summary

### Summary

`parse()` has a quadratic-time path in `parseKey`, reachable on default options with ordinary valid input. For every key line and table-header line, `parseKey` (dist/struct.js, lines 58 and 86) finds the dotted-key separator with `ctx.s.indexOf('.', ctx.p)`, where `ctx.s` is the whole document. When a key has no `.` ahead of it, that search runs all the way to the end of the input, and the result is then clamped back to the line terminator `endPtr` - so everything scanned past the current line is wasted. `parseKey` runs once per line, so a document of N dot-free keys costs O(n^2).

The most ordinary TOML shape triggers it: a flat list of `key = value` lines, or a repeated `[[a]]` table. No dotted keys, no special options, valid input throughout.

### Proof of concept

```js import { parse } from 'smol-toml'

let doc = '' for (let i = 0; i < 256000; i++) doc += 'k' + i + ' = 1\n'

console.time('parse') parse(doc) // ~2.8 MB of valid TOML, default options console.timeEnd('parse') ```

Doubling the line count roughly quadruples the time:

| lines | size | parse() | |---|---|---| | 32k | 0.3 MB | 0.3 s | | 64k | 0.7 MB | 1.0 s | | 128k | 1.4 MB | 3.5 s | | 256k | 2.8 MB | 14 s |

### Impact Any service that runs `parse()` on attacker-supplied TOML can be stalled. The work is synchronous, so it blocks the whole event loop, and the quadratic is unbounded: a ~7 MB body pins a core for about a minute, larger bodies for several.

### Patches Version 1.9.0 uses a different implementation for parsing keys which is strictly linear.

### Workarounds Limit the maximum document size accepted when parsing arbitrary documents.