CR8-USD <-> MUSD parity swap router for cross-vertical settlement

[abhicris]

Problem

Two stables ship on Create Protocol L1: CR8-USD (cross-vertical settlement, gas) and MUSD (music-vertical settlement, DSP payouts). Agents that span verticals — e.g. an IPTO query that returns a Muzix-indexed catalog and triggers a royalty payout — need to settle one leg in CR8-USD and the other in MUSD without round-tripping through an external DEX.

We need an in-protocol parity swap router that gives 1:1 conversion (modulo a small protocol fee) so settlement is atomic and the agent does not eat slippage on a single-block flow.

Proposed Approach

1. contracts/StableSwapRouter.sol holds reserves of both stables and mints/burns at parity:
   • swapCR8USDtoMUSD(uint256 amount, address to) — burns CR8-USD, mints MUSD, takes a protocol fee in bps (default 5 bps, configurable by Council vote).
   • swapMUSDtoCR8USD(uint256 amount, address to) — inverse.
2. Reserve invariant: total CR8-USD + MUSD supply across both is preserved (no inflation). Router holds no excess; mints/burns one-for-one.
3. Fee accrual: protocol fee accrues to a treasury contract per the standard waterfall (60/30/10 validator / treasury / public-goods split).
4. Anti-griefing: rate-limit per address (e.g. $100K notional per epoch by default; raised by Council vote for known integrators).
5. switchboard/swap.py thin client wrapper so AgentEscrow can call the router inline as part of the settle flow.

Acceptance Criteria

• Router contract deployed and audited (auditor TBD)
• Parity invariant verified on every swap (no rounding leak)
• Per-address rate limit enforced
• AgentEscrow integration: a single settle call can swap legs and release in the target stable
• Fee accrual visible in treasury balance after a representative test workload

References

• internal: see Create Protocol PQ L1 architecture RFC section 4 (tokens) and stress-test 2026-06-01 section 2.1
• Muzix MUSD design doc