Create a bank securities snapshot

Verified non-API-key staff/service with ledger:securities:read captures immutable source evidence under exact actor/key/input replay. This read authority cannot issue trades. ObservationDate is the actual current source date; requested BusinessDate describes close coverage, not reconstructed historical inventory. Snapshot retains original control/receipt evidence, assigned journal IDs, keys, actors, posting dates and commit times plus exact fingerprints including hold release; actual managed balances and dealer registry checks remain explicit. Day-close coverage needs the requested day's evidenced value and accrual through exclusive boundary D+1. Independent actual master-statement reconciliation remains a blocker until separately proven; provider signatures alone cannot make the book ready. Complete response is bounded at 16 MiB and never silently truncated. Customers and API keys are refused even locally.