Add Profile::excerpt() for plain text preview generation

[dereuromark]

Summary

Adds a new Profile::excerpt() static method for generating plain text previews/snippets from Djot content.

Use Cases

1. Search Result Snippets

When indexing content for search, you want to show a preview of the actual content without quoted text or code blocks:

$converter = new DjotConverter(profile: Profile::excerpt());
$html = $converter->convert($document->content);
$snippet = mb_substr(trim(strip_tags($html)), 0, 200) . '...';

2. Social Media Previews (Open Graph)

For meta descriptions, you want the author's text, not quoted material:

// Blog post with quote at the start
$content = <<<'DJOT'
> "The best code is no code at all." — Jeff Atwood

In this article, I'll explain why simplicity matters...
DJOT;

$converter = new DjotConverter(profile: Profile::excerpt());
$text = trim(strip_tags($converter->convert($content)));
// Result: "In this article, I'll explain why simplicity matters..."
// (Quote is stripped)

3. Forum/Comment Thread Previews

Show the user's actual response, not what they're quoting:

// Forum reply with nested quotes
$reply = <<<'DJOT'
> @user123 wrote:
> I think the API needs improvement.

Actually, the API is well-designed. Here's why...
DJOT;

$converter = new DjotConverter(profile: Profile::excerpt());
$preview = trim(strip_tags($converter->convert($reply)));
// Result: "Actually, the API is well-designed. Here's why..."

4. RSS Feed Summaries

Generate clean summaries without code blocks or images:

$converter = new DjotConverter(profile: Profile::excerpt());
$html = $converter->convert($article->body);
$summary = html_entity_decode(trim(strip_tags($html)));

5. Email Subject Lines from Content

Extract meaningful text for automated emails:

$converter = new DjotConverter(profile: Profile::excerpt());
$text = trim(strip_tags($converter->convert($notification->message)));
$subject = mb_substr($text, 0, 50);

What Gets Stripped

Element Type	Reason
Blockquotes	Quoted content represents others' text, not the author's
Code blocks	Technical details not suitable for previews
Images	Cannot be represented in plain text
Tables	Complex structure doesn't translate to excerpts
Footnotes	Reference material not needed in previews
Raw HTML	Security concern and doesn't render as text
Math/Symbols	Don't translate well to plain text

What Gets Kept

• Paragraphs, headings, lists (structure for text extraction)
• Inline formatting (bold, italic, etc.) - renders as HTML, stripped by strip_tags()
• Links - link text is preserved after strip_tags()

Test Plan

• Added 13 new test cases covering excerpt profile functionality
• Added use case tests for search snippets and Open Graph
• Updated existing profile name/description tests
• All 94 profile tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (1ea357f) to head (32f1724).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #55      +/-   ##
============================================
+ Coverage     93.40%   93.44%   +0.04%     
- Complexity     1928     1929       +1     
============================================
  Files            74       74              
  Lines          5183     5221      +38     
============================================
+ Hits           4841     4879      +38     
  Misses          342      342              

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[dereuromark]

Closing this - the existing API already supports this use case perfectly with Profile::minimal()->denyBlock([...])->onDisallowed(Profile::ACTION_STRIP). A built-in profile isn't necessary for such a simple configuration.