DESK-WIDE DAILY LOSS LIMIT (Wave A) — set (or clear with null) the dollars this DESK may lose in one ET session before every bot on it stops opening. Session-owner scoped like the other desk mutations; 404 if the desk is not the caller's (cloak).

WHAT MAKES IT DIFFERENT from the per-bot risk.daily_loss_usd in a spec: this one counts the WHOLE DESK's realized P&L for the session, so one bot's bad morning can stop every other bot on the account. That is the point — every other limit we ship is a bot talking to itself, and fifty individually well-behaved bots can still lose the account.

Each bot READS this state on its own walk and derives the desk's P&L fresh from the desk's own closed rows: nothing is written, nothing is reserved, and a new ET session starts clean with no resume step. It pauses ENTRIES ONLY — marks, exits and settlement never consult it.