Performance: isVoidElement creates map on every call

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## Description

The `isVoidElement()` function creates a new map allocation on every call instead of using a package-level constant.

## Location

- `html/parser.go:131-149`

## Problem

```go
func isVoidElement(tagName string) bool {
voidElements := map[string]bool{ // Allocated every call!
"area": true,
"base": true,
// ... 14 entries
}
return voidElements[tagName]
}
```

This function is called for every start tag during HTML parsing, causing unnecessary allocations.

## Suggested Fix

Move the map to package level:

```go
var voidElements = map[string]bool{
"area": true,
"base": true,
// ...
}

func isVoidElement(tagName string) bool {
return voidElements[tagName]
}
```

## Impact

Minor but easy fix. Reduces GC pressure during HTML parsing, especially for large documents.

Open html/parser.go:131-149 and inspect isVoidElement and its per-call map allocation. Move the existing void-element lookup to package scope as described, then verify that HTML parsing still recognizes the same void elements without allocating the map on each call.

Written by the indexing model from the issue text.