feat(checkout): add Checkout orchestrator for one-call agent-commerce flow (#45)

## 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):

```python
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):

```python
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

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

Author: vvillait88

agentscore_commerce/__init__.py

@@ -6,16 +6,40 @@
     agentscore_commerce.discovery  - probe + .well-known/mpp.json + llms.txt + OpenAPI snippets
     agentscore_commerce.challenge  - 402-body builders
     agentscore_commerce.stripe_multichain - multichain PaymentIntent helpers
+    agentscore_commerce.checkout   - high-level Checkout orchestrator
     agentscore_commerce.api        - AgentScore SDK re-export
 """
 
 from importlib.metadata import PackageNotFoundError
 from importlib.metadata import version as _pkg_version
 
+from agentscore_commerce.checkout import (
+    Checkout,
+    CheckoutContext,
+    CheckoutRailSpec,
+    CheckoutRequest,
+    CheckoutResult,
+    MppxComposeOutcome,
+    PricingResult,
+    SettleOutcome,
+)
+
 try:
     __version__ = _pkg_version("agentscore-commerce")
 except PackageNotFoundError:
     # Editable install or pre-build state — fall back to a sentinel so consumers
     # don't crash on a missing dist-info dir. Real version always comes from
     # pyproject.toml at install time.
     __version__ = "0.0.0+local"
+
+__all__ = [
+    "Checkout",
+    "CheckoutContext",
+    "CheckoutRailSpec",
+    "CheckoutRequest",
+    "CheckoutResult",
+    "MppxComposeOutcome",
+    "PricingResult",
+    "SettleOutcome",
+    "__version__",
+]