well-known: add /.well-known/ard.json (ARD v0.91 canonical path)

[flober81]

Adds /.well-known/ard.json next to /.well-known/ai-catalog.json (#201).

Why: ARD spec v0.91 (ards-project/ard-spec, 2026-08-26, "promote the draft, single well-known path") makes /.well-known/ard.json the canonical location: "A consumer resolving a domain's entries MUST fetch /.well-known/ard.json, and MUST honour a rel="ard" link." ai-catalog.json and rel="ai-catalog" are now the predecessor. Lighthouse 13.5's ARD gatherer still resolves the predecessor path (GoogleChrome/lighthouse#17251 tracks updating it to v0.91), so the #201 entry stays as it is; recording the new path separately is what lets HTTP Archive see whether publishers move.

What is recorded: entries_count and entry_types (capped at 20), keyed by /.well-known/ard.json. The v0.91 manifest only requires entries ("any other top-level members are transport-defined and ignored"), so spec_version and has_host are not recorded for the new path. A <link rel="ard"> pointing elsewhere wins and is recorded as catalog_url, mirroring #201; not found is just {"found": false}. Entry parsing is shared with the ai-catalog block through a small helper; the ai-catalog output is unchanged (checked old vs. new over nine cases). Invalid JSON or an HTML catch-all page yields the default values, no error.

---

Test websites:

• https://btlabs.dev/ (serves both paths with identical JSON, <link rel="ard"> in the head)
• https://example.com/ (negative control, expected found: false)

[github-actions]

https://almanac.httparchive.org/en/2022/

WPT result details

Changed custom metrics values:

{
  "_well-known": {
    "/.well-known/assetlinks.json": {
      "found": false
    },
    "/.well-known/apple-app-site-association": {
      "found": false
    },
    "/.well-known/related-website-set.json": {
      "found": false
    },
    "/.well-known/privacy-sandbox-attestations.json": {
      "found": false
    },
    "/.well-known/gpc.json": {
      "found": false
    },
    "/.well-known/web-identity": {
      "found": false
    },
    "/.well-known/passkey-endpoints": {
      "found": false
    },
    "/.well-known/webauthn": {
      "found": false
    },
    "/.well-known/ucp": {
      "found": false
    },
    "/.well-known/email-verification": {
      "found": false
    },
    "/robots.txt": {
      "found": true,
      "data": {
        "matched_disallows": {}
      }
    },
    "/.well-known/ai-catalog.json": {
      "found": false
    },
    "/.well-known/ard.json": {
      "found": false
    },
    "/.well-known/security.txt": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": true,
        "url": "https://almanac.httparchive.org/.well-known/security.txt/",
        "content_type": "text/html; charset=utf-8"
      }
    },
    "/.well-known/change-password": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": true,
        "url": "https://almanac.httparchive.org/.well-known/change-password/"
      }
    },
    "/.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200/": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": false,
        "url": "https://almanac.httparchive.org/.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200/"
      }
    }
  }
}

https://btlabs.dev

WPT result details

Changed custom metrics values:

{
  "_well-known": {
    "/.well-known/assetlinks.json": {
      "found": false
    },
    "/.well-known/apple-app-site-association": {
      "found": false
    },
    "/.well-known/related-website-set.json": {
      "found": false
    },
    "/.well-known/privacy-sandbox-attestations.json": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/gpc.json": {
      "found": true,
      "data": {
        "gpc": true
      }
    },
    "/.well-known/web-identity": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/passkey-endpoints": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/webauthn": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/ucp": {
      "found": false
    },
    "/.well-known/email-verification": {
      "error": "signal is aborted without reason"
    },
    "/robots.txt": {
      "found": true,
      "data": {
        "matched_disallows": {
          "*": [
            "/admin"
          ]
        }
      }
    },
    "/.well-known/ai-catalog.json": {
      "found": true,
      "data": {
        "spec_version": "1.0",
        "has_host": true,
        "entries_count": 3,
        "entry_types": [
          "application/openapi+yaml",
          "application/mcp-server-card+json"
        ]
      }
    },
    "/.well-known/ard.json": {
      "found": true,
      "data": {
        "entries_count": 3,
        "entry_types": [
          "application/openapi+yaml",
          "application/mcp-server-card+json"
        ]
      }
    },
    "/.well-known/security.txt": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/change-password": {
      "error": "signal is aborted without reason"
    },
    "/.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200/": {
      "error": "signal is aborted without reason"
    }
  }
}

https://example.com

WPT result details

Changed custom metrics values:

{
  "_well-known": {
    "/.well-known/assetlinks.json": {
      "found": false
    },
    "/.well-known/apple-app-site-association": {
      "found": false
    },
    "/.well-known/related-website-set.json": {
      "found": false
    },
    "/.well-known/privacy-sandbox-attestations.json": {
      "found": false
    },
    "/.well-known/gpc.json": {
      "found": false
    },
    "/.well-known/web-identity": {
      "found": false
    },
    "/.well-known/passkey-endpoints": {
      "found": false
    },
    "/.well-known/webauthn": {
      "found": false
    },
    "/.well-known/ucp": {
      "found": false
    },
    "/.well-known/email-verification": {
      "found": false
    },
    "/robots.txt": {
      "found": false
    },
    "/.well-known/ai-catalog.json": {
      "found": false
    },
    "/.well-known/ard.json": {
      "found": false
    },
    "/.well-known/security.txt": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": false,
        "url": "https://example.com/.well-known/security.txt",
        "content_type": "text/html"
      }
    },
    "/.well-known/change-password": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": false,
        "url": "https://example.com/.well-known/change-password"
      }
    },
    "/.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200/": {
      "found": false,
      "data": {
        "status": 404,
        "redirected": false,
        "url": "https://example.com/.well-known/resource-that-should-not-exist-whose-status-code-should-not-be-200/"
      }
    }
  }
}