fix(seo): shorten meta descriptions to 110-160 chars

Shorten meta descriptions to prevent truncation in search snippets.

9 pages updated:
- overview: 161 → 130 chars
- hoisting: 188 → 145 chars
- proxy-reflect: 179 → 139 chars
- tagged-template-literals: 168 → 124 chars
- intersection-observer: 175 → 136 chars
- typed-arrays-arraybuffers: 164 → 120 chars
- custom-events: 161 → 125 chars
- performance-observer: 196 → 130 chars
- mutation-observer: 178 → 120 chars

Per ahrefs SEO recommendation: descriptions should be 110-160 chars.