add compile-fail test suite for typed API

[dannywillems]

Description

The typed API's main value proposition is compile-time validation of HTML nesting. There is only one `compile_fail` doctest, but no systematic compile-fail tests for:

• Putting `
  ` inside `

  `

• Putting `
• ` directly in `
  ` (should require `

  ` or `
  `)
  1. Putting `` directly in `` (should require ``)
  2. Text in void elements

  A `trybuild` or doctest-based compile-fail suite would document and enforce the type constraints.