fix(mpp-seller): bind instrument and Stripe receipts

[nkavian]

Summary

Reject Stripe and InFlow instrument receipts with a different method, a different challenge identifier, or no challenge identifier. Matching receipts retain their fields. Balance, Tempo, subscriptions and CARD behavior are unchanged.

Add protected-route regression tests, the shared Stripe adapter, an exact contract pin and hosted Stripe conformance runs. Include a patch changeset for the Seller package.

Verification

• pnpm verify passed, including type checks, lint, tests, TAP conformance, changed-line coverage, TypeDoc and build.
• pnpm check-exports and pnpm verify-publish passed.
• Seller tests passed on Node 22 and 24.
• Shared runtime, MPP, Stripe and x402 cases passed on both runtimes.

Tests use synthetic local platform responses, not live payments. The instrument receipt check does not enable instrument settlement on the platform.

Merge inflowpayai/inflow-specs#23 first. This PR pins its contract commit; the current-main drift job also requires that contract to be merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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