fix: stricter type narrowing for head schema

[harlan-zw]

🔗 Linked issue

Related to #527

❓ Type of change

• 📖 Documentation
• 🐞 Bug fix
• 👌 Enhancement
• ✨ New feature
• 🧹 Chore
• ⚠️ Breaking change

📚 Description

The head schema types were too loose — <base> accepted empty objects, <link> didn't narrow by rel, and attributes like lang/inputmode/target were plain strings with no autocomplete. This tightens type narrowing across the schema so that discriminated unions (e.g. Link by rel, Base requiring href or target, Meta by name/property/http-equiv) provide better IDE autocomplete and catch invalid combinations at the type level.

Examples of code that now errors

Empty <base> (must provide href or target)

// ❌ TS error — Base requires at least `href` or `target`
useHead({ base: {} })

// ✅
useHead({ base: { href: '/' } })
useHead({ base: { target: '_blank' } })

Mixing name + property on <meta>

// ❌ TS error — `property` is `never` on NameMeta
useHead({
  meta: [{ name: 'description', property: 'og:title', content: 'Hello' }]
})

// ✅ separate entries
useHead({
  meta: [
    { name: 'description', content: 'Hello' },
    { property: 'og:title', content: 'Hello' },
  ]
})

charset meta with content

// ❌ TS error — `content` is `never` on CharsetMeta
useHead({
  meta: [{ charset: 'utf-8', content: 'text/html' }]
})

// ✅
useHead({
  meta: [{ charset: 'utf-8' }]
})

External script with textContent

// ❌ TS error — `textContent` is `never` when `src` is present
useHead({
  script: [{ src: '/app.js', textContent: 'console.log("hi")' }]
})

// ✅ pick one
useHead({ script: [{ src: '/app.js' }] })
useHead({ script: [{ textContent: 'console.log("hi")' }] })

JSON-LD with src

// ❌ TS error — `src` is `never` on JsonLdScript
useHead({
  script: [{ type: 'application/ld+json', src: '/schema.json' }]
})

// ✅
useHead({
  script: [{ type: 'application/ld+json', textContent: { '@type': 'WebSite' } }]
})

<link rel="mask-icon"> missing required color

// ❌ TS error — `color` is required on MaskIconLink
useHead({
  link: [{ rel: 'mask-icon', href: '/icon.svg' }]
})

// ✅
useHead({
  link: [{ rel: 'mask-icon', href: '/icon.svg', color: '#000' }]
})

<noscript> with both textContent and innerHTML

// ❌ TS error — can't provide both (one must be `never`)
useHead({
  noscript: [{ textContent: 'JS disabled', innerHTML: '<p>JS disabled</p>' }]
})

// ✅ pick one
useHead({ noscript: [{ textContent: 'JS disabled' }] })
useHead({ noscript: [{ innerHTML: '<p>JS disabled</p>' }] })

<link> without href (GenericLink now requires it)

// ❌ TS error — `href` is required on GenericLink
useHead({
  link: [{ rel: 'stylesheet' }]
})

// ✅
useHead({
  link: [{ rel: 'stylesheet', href: '/style.css' }]
})