[pack-refresh] javascript 26.4 released — review docs/languages/javascript.md

[github-actions]

The pack-freshness CI detected a new stable version of javascript.

• Last reviewed: 25.9
• Current stable: 26.4 (raw: 26.4.0)
• Pack file: docs/languages/javascript.md

What to do

1. Read the upstream release notes and identify:
   • New stdlib additions that deserve a new idiom entry
   • Syntax changes or deprecations that deserve a new anti-pattern or mental-model delta
   • New challenge seed opportunities
2. Update docs/languages/javascript.md with the changes
3. Mirror idiom/seed changes into .claude/skills/challenge/SKILL.md
4. Update the pack's last_reviewed_against frontmatter field to "26.4"
5. Open a PR titled "refresh javascript pack against 26.4"

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Opened automatically by .github/workflows/pack-freshness-check.yml