Performance: Classes() parses class string on every call

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## Description

The `dom.Node.Classes()` method parses the class attribute string every time it's called, with no caching.

## Location

- `dom/node.go:85-101` - `Classes()` method
- `style/style.go:292` - Called in hot path during selector matching

## Problem

```go
func (n *Node) Classes() []string {
class := n.GetAttribute("class")
// Parses class string every time - O(n) string parsing
classes := []string{}
start := 0
for i := 0; i <= len(class); i++ {
// ...
}
return classes
}
```

This is called for every selector match attempt, meaning the same class string gets parsed hundreds or thousands of times for a single element.

## Suggested Improvements

1. **Cache parsed classes on the Node struct** - Store `parsedClasses []string` and invalidate on attribute change
2. **Use `strings.Fields()`** - More efficient than manual parsing
3. **Lazily initialize and cache** - Parse once on first access

## Impact

For a page with 100 elements and 50 CSS rules with class selectors, this causes ~5000 redundant string parsing operations.

Start in dom/node.go:85-101 to inspect Classes(), then trace attribute changes on Node and its use from style/style.go:292. Choose a caching approach that remains valid after class attribute updates, and confirm selector matching no longer reparses the same class string on every attempt.

Written by the indexing model from the issue text.