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The special discovery-index account was keyed as "@index", which the consumer's normalizeUsername collapses to "index" — colliding with a real user named "index" (who could then hijack/tamper the global index document). Introduce a reserved-account namespace (a sentinel key unreachable by any typed username) so the index account can never collide with user input. Test proves a user named "index"/"@index" resolves to a DIFFERENT document than the reserved index account. 80 tests pass; tsc rc=0. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>