Add attributed string postprocessor

[tseylerd]

Problem

MarkupParser only runs when the input string changes, so there is no way to add attributes whose source is external to the content (for example, highlight search occurences). Highlighting (or any content-independent customization) currently requires re-parsing and re-rendering which is very inefficient.

Solution

Introduces an AttributedStringPostprocessor protocol and .textual.attributedStringPostprocessor(_:) modifier that lets callers transform the parsed AttributedString independently of markup content. Applies to both InlineText and StructuredText.

The postprocessor runs after parsing and attachment resolution, and is re-invoked whenever either the parsed attributed string or the postprocessor itself changes. Equality (from Hashable) drives onChange-based invalidation, so conforming types reflect state affecting output in their ==.

struct SearchHighlighter: AttributedStringPostprocessor {
  let query: String

  func postprocess(input: AttributedString) -> AttributedString {
    var output = input
    var start = output.startIndex

    while let range = output[start...].range(of: query, options: .caseInsensitive) {
      output[range].backgroundColor = .yellow.opacity(0.4)
      start = range.upperBound
    }

    return output
  }
}

InlineText(markdown: "Hello, world")
  .textual.attributedStringPostprocessor(SearchHighlighter(query: "world"))

Alternatives Considered

• Implementing custom MarkupParser: inefficient, requires re-parsing every time without string changes.

Testing

• New tests pass: swift test --filter AttributedStringPostprocessorTests (3 tests).
• Demo app "Postprocessor" page.