HTML: Fix importing docs which contain attribute values prefixed with newlines

[roblillack]

This PR:

• Hardens the HTML tokenizer so whitespace between an attribute’s = and the opening quote no longer drops the parser out of “attribute value” mode, preventing quoted hrefs from accidentally absorbing the surrounding text
• Adds a FreeBSD 15.0 release-notes fixture plus snapshots
• Adds a single Markdown round-trip exclusion so the release-notes snapshot (which currently hits known importer gaps) doesn’t break the rest of the round-trip suite

Fixes #29