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": "stable-thread-ordering-by-date",
  "description": "Independent threads are ordered by earliest visible message date.",
  "spec": {
    "source": "public API contract",
    "url": "https://www.rfc-editor.org/rfc/rfc5256",
    "behavior": "flat public output is deterministic across input order"
  },
  "input": [
    {
      "id": "<later@example>",
      "message_id": "<later@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Later",
      "date": "2026-05-15T10:00:00Z"
    },
    {
      "id": "<earlier@example>",
      "message_id": "<earlier@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Earlier",
      "date": "2026-05-15T09:00:00Z"
    }
  ],
  "expected": {
    "threads": [
      {
        "root": "<earlier@example>",
        "messages": [
          "<earlier@example>"
        ]
      },
      {
        "root": "<later@example>",
        "messages": [
          "<later@example>"
        ]
      }
    ]
  }
}