Confirm primitives in create-protocol/cr8/specs/switchboard-integration.md §4 are stable surface

[abhicris]

The newly merged create-protocol/cr8 switchboard-integration spec §4 enumerates six switchboard primitives that the Create Protocol registry depends on. This issue tracks the work to:

1. Confirm each is currently part of switchboard's stable public surface (i.e. won't change without a MAJOR bump).
2. Document any gaps and ship them.
3. Add a "create-protocol-stable" doc page that pins this surface and links back to the spec by commit-pinned URL.

Primitives in scope (per spec §4)

Primitive	Switchboard surface
MPC wallet provisioning	`switchboard.provision_wallet(quorum, recovery_set) -> Address`
Threshold signing	`switchboard.sign(wallet_id, payload) -> Signature`
Key rotation	`switchboard.rotate(wallet_id, new_quorum) -> Address`
x402 metering	`switchboard.meter(session_id, rate) -> Receipt`
A2A counterparty handshake	`switchboard.a2a_handshake(peer) -> Channel`
Recovery quorum lookup	`switchboard.recovery_quorum(wallet_id) -> Set`

Acceptance

• Each row in the table above either passes a smoke test or has a sub-issue tracking the gap.
• `docs/create-protocol-stable.md` lists the stable surface + version contract.
• README adds a one-line link to that doc.

Spec source

• `create-protocol/cr8/specs/switchboard-integration.md`