docs: add alternative nanopayment architecture reference (vault + batched voucher settlement)

[osr21]

Summary

Adds docs/ALTERNATIVE_ARCHITECTURES.md, a short reference comparing this
repo's Gateway x402 batching model to an alternative vault + EIP-712 session
voucher + keeper-batched-settlement design (NanopayVault, from
osr21/arc-stablecoin-dapp),
built while integrating agent-to-agent nanopayments on Arc.

Why

Both approaches solve the same "avoid gas-per-micropayment" problem but make
different tradeoffs (managed facilitator vs. self-hosted keeper batching,
per-request settlement vs. per-session voucher + off-chain metering). This is
purely additive documentation — no changes to any existing code path — meant
as a reference point for anyone comparing designs for agent-to-agent
micropayments on Arc, not a suggestion to change this repo's approach.

[osr21]

Great reference — we've extended the same project with a third architecture variant since this PR was opened.

NanopayP2P (NanopayP2P.sol, deployed on Arc Testnet at 0x65a2FD4E63767Cee00d2Aa08cba2feBFb1a21a9f) removes the operator/keeper role entirely — the seller claims directly, no relay key anywhere in the system.

	arc-nanopayments (this repo)	NanopayVault (from this PR)	NanopayP2P (new)
Token custody	Gateway facilitator	On-chain vault contract	On-chain channel contract
Buyer authorizes	x402 PaymentRequirements per request	EIP-712 SessionVoucher (zero gas)	EIP-712 PaymentVoucher (zero gas)
Settlement trigger	Per-request (facilitator)	Keeper batch (settleBatch())	Seller calls claim() directly
Trusted operator / relay key	Circle Gateway	✅ required	❌ none
Server private key	✅ seller wallet	✅ operator relay key	❌ none
Withdrawal attack surface	✅ (see issue #29)	✅ (operator key exposure)	❌ eliminated
Cumulative payment ceiling	Per-request	Per-session maxBudget	Per-channel deposit

The elimination of the server key is what resolves the attack in issue #29 — there's no private key on the server and no withdrawal endpoint to hit. Worth noting as a fourth row in ALTERNATIVE_ARCHITECTURES.md if you extend that doc, since it represents a different point on the trust/UX tradeoff spectrum.

Source: osr21/arc-stablecoin-dapp — contracts/src/NanopayP2P.sol and artifacts/arc-dapp/src/pages/nanopay.tsx.