fix(auth): materialize trust superuser identity - #198

Summary

• materialize the configured trust-mode superuser as a durable catalog identity before listeners start
• remove empty-store and fabricated user_id = 0 trust authority across pgwire, native, HTTP/WS, and sync
• make trust/password superuser bootstrap failure-atomic by committing the user and next-user-ID counter together
• preserve existing regular-user credentials and stable identity while rejecting service-account conflicts
• add cross-protocol, restart, ownership-integrity, transition, and rollback regression coverage

Validation

• cargo nextest run -p nodedb --lib -E 'test(credential::store) | test(backward_compat_stored_user_defaults) | test(control::server::sync::session::handshake)'
• cargo nextest run -p nodedb --test pgwire_auth_wire --test native_handshake_e2e --test http_route_authentication --no-fail-fast
• HTTP/WS focused suite: 69 passed
• pgwire/tenant/startup focused suite: 75 passed
• cargo clippy -p nodedb --all-targets -- -D warnings
• cargo fmt --all --check

Closes #195