feat(checkout): add Checkout orchestrator for one-call agent-commerce flow

[vvillait88]

Summary

High-level orchestrator that collapses 402-emit + verify+settle into a single await checkout.handle() call. Services every merchant shape via the same primitive:

Axis	Options
Rail mix	x402-only / MPP-only / both
Custody	self-custody (chain) / Stripe (custodial) / mixed
Identity	gated (AgentScoreCore upstream) / ungated
Use case	goods seller (per-order) / API seller (per-call)

Common usage

Goods seller (full agent-commerce):

checkout = Checkout(
    rails={
        "tempo": TempoRailSpec(recipient=...),
        "x402_base": X402BaseRailSpec(recipient=...),
        "stripe": StripeRailSpec(profile_id=...),
    },
    url=APP_URL,
    compute_pricing=lambda ctx: PricingResult(amount_usd=cart_total(ctx.body)),
    mint_recipients=lambda ctx: stripe_multichain_addresses_for(ctx),
    on_settled=lambda ctx, outcome: persist_order(ctx.reference_id, ctx.body, outcome),
    compose_mppx=lambda ctx: mppx_compose(mppx, ctx.request),
    x402_server=x402,
)

API seller (per-call x402):

checkout = Checkout(
    rails={"x402_base": X402BaseRailSpec(recipient=TREASURY)},
    url=APP_URL,
    compute_pricing=lambda ctx: PricingResult(amount_usd=0.01),
    on_settled=lambda ctx, _: {"data": await run_api(ctx.body)},
    x402_server=x402,
)

Design notes

• Framework-neutral: handle() takes CheckoutRequest, returns CheckoutResult (body + headers + status + reference_id + settled). Merchant wraps in their framework's response.
• Domain-neutral: reference_id is a UUID; goods merchants persist as order id, API merchants treat as request id.
• x402 base network is auto-derived from rails["x402_base"].network — no duplicate kwarg.
• CheckoutRequest.raw is an escape hatch for compose_mppx hooks that need the framework's native request object.
• Flattening rule respected: __init__ is flat-kwargs with * keyword-only marker; all dataclasses are value types (data shapes for inputs/outputs), not parameter-bundle wrappers.

Test plan

• pytest green (1076 tests pass; coverage 95.17%)
• ty check clean
• ruff check clean
• vulture clean
• 12-test matrix in tests/test_checkout.py covers every flexibility axis