Fix model-building and rendering defects (#56 #57 #58 #59 #62 #65)

[wolfgang-aura]

Fixes #56
Fixes #57
Fixes #58
Fixes #59
Fixes #62
Fixes #65

One commit and one regression check in test/dom.test.mjs per issue. No version bump.

• Completeness verdict is wrong both ways: truncated main post reads COMPLETE, caption-less media post reads INCOMPLETE #56: a top-level truncation-notice counts as a truncated main post. A caption-less post with media, a poll or a quote no longer reads as missing main text.
• .llm.md loses line breaks inside posts #57: .llm.md paragraphs keep line breaks. Post text cannot open its own Markdown heading, setext underline or code fence.
• Article export drops repeated headings and paragraphs #58: article export drops a text block only when it equals the previous text block.
• Post author handle is read from the display name when the name contains @word #59: author handle comes from the profile link href. The last @word is the fallback when there is no link.
• Multi-line alt text breaks .llm.md structure and bundle image embeds #62: multi-line alt text folds to one line. Bundle embeds escape brackets, and the quoted-post video bullet keeps its closing bracket.
• Quote dedupe merges quotes and drops images across thread posts #65: quote dedupe runs per thread post.

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