[site] add BingSiteAuth.xml for Bing Webmaster Tools verification

Bing verifies site ownership by fetching /BingSiteAuth.xml from the site root.
public/ is copied verbatim into dist/ by vite, so the file lands at
https://onchainattack.org/BingSiteAuth.xml on the next Pages deploy. The token
is a public ownership marker, not a secret.

Bing has never had anything to index here — every route was an empty container
until the pre-render landed in 7d8528b — so verifying now lets its crawler pick
up the 1000-URL sitemap directly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PGb1rogz5B1rkcUzU2tJv4

Author: iZonex

public/BingSiteAuth.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<users>
+	<user>4C737FD2FDFEEDAC5E65C27D21A9FFDD</user>
+</users>