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": "two-independent-threads",
  "description": "Unrelated reference chains remain separate threads.",
  "spec": {
    "source": "RFC 5256 / JWZ",
    "url": "https://www.rfc-editor.org/rfc/rfc5256",
    "behavior": "containers without shared ancestors produce separate thread roots"
  },
  "input": [
    {
      "id": "<a-root@example>",
      "message_id": "<a-root@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Topic A",
      "date": "2026-05-15T09:00:00Z"
    },
    {
      "id": "<a-reply@example>",
      "message_id": "<a-reply@example>",
      "in_reply_to": "<a-root@example>",
      "references": [
        "<a-root@example>"
      ],
      "subject": "Re: Topic A",
      "date": "2026-05-15T09:05:00Z"
    },
    {
      "id": "<b-root@example>",
      "message_id": "<b-root@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Topic B",
      "date": "2026-05-15T10:00:00Z"
    },
    {
      "id": "<b-reply@example>",
      "message_id": "<b-reply@example>",
      "in_reply_to": "<b-root@example>",
      "references": [
        "<b-root@example>"
      ],
      "subject": "Re: Topic B",
      "date": "2026-05-15T10:05:00Z"
    }
  ],
  "expected": {
    "threads": [
      {
        "root": "<a-root@example>",
        "messages": [
          "<a-root@example>",
          "<a-reply@example>"
        ]
      },
      {
        "root": "<b-root@example>",
        "messages": [
          "<b-root@example>",
          "<b-reply@example>"
        ]
      }
    ]
  }
}