mail-threading 0.1.3

JWZ email threading (RFC 5256). Reconstruct message threads from References/In-Reply-To headers, with subject fallback for broken clients.
Documentation 
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{
  "name": "subject-fallback-attaches-to-header-thread",
  "description": "A headerless subject reply attaches to an existing header-backed thread.",
  "spec": {
    "source": "JWZ",
    "url": "https://www.jwz.org/doc/threading.html",
    "behavior": "subject fallback prefers an existing thread with the same base subject"
  },
  "input": [
    {
      "id": "<topic-root@example>",
      "message_id": "<topic-root@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Topic",
      "date": "2026-05-15T09:00:00Z"
    },
    {
      "id": "<topic-reply@example>",
      "message_id": "<topic-reply@example>",
      "in_reply_to": "<topic-root@example>",
      "references": [
        "<topic-root@example>"
      ],
      "subject": "Re: Topic",
      "date": "2026-05-15T09:05:00Z"
    },
    {
      "id": "<topic-headerless@example>",
      "message_id": "<topic-headerless@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "SV: Topic",
      "date": "2026-05-15T09:10:00Z"
    }
  ],
  "expected": {
    "threads": [
      {
        "root": "<topic-root@example>",
        "messages": [
          "<topic-root@example>",
          "<topic-reply@example>",
          "<topic-headerless@example>"
        ]
      }
    ]
  }
}