MPP sessions adapter: open Tempo session, stream micro-pays under a budget cap

[abhicris]

Why

MPP (Stripe × Tempo, mainnet 2026-03-18) introduces a session primitive: an agent authorises a spending cap upfront and streams micro-payments without an on-chain tx per request. This is the right primitive for high-frequency agent traffic where x402 per-request becomes cost-prohibitive even at $0.001.

Switchboard's gas-budget tracker (#5 / merged #14) already understands rolling spending caps. An MPP-session adapter is a natural composition: switchboard owns the cap, MPP owns the session.

Scope

• switchboard.mpp.session — Python client around Tempo's MPP API: open / charge / close.
• Bind OpenSession.limit to the existing GasBudgetTracker daily limit so a single source-of-truth caps the agent's spend.
• On session close, write the settled net amount into the gas-budget ledger so cross-protocol caps stay honest.
• Optional: SPT bridge — accept a Stripe Shared Payment Token instead of native USDC for the funding side. Document the trust trade-off.

Deliverables

• Module + tests against a recorded MPP API trace (no live mainnet calls in CI).
• examples/mpp_streaming_inference.py — agent paying for streamed inference under a 10 USD/day cap.
• Docs section + dashboard tab update.

Non-goals

• Authoring our own session-server. We're a client of MPP.