docs(spec): align scheme_exact_algo with v2 amount and verification

[temptemp3]

Summary

Updates specs/schemes/exact/scheme_exact_algo.md for Coinbase review alignment (x402 v2):

• Use amount consistently (not maxAmountRequired).
• Document verification order: cross-validation (x402Version, scheme, network, accepted vs 402), genesis hash (gh) vs CAIP-2, asset amount/receiver/asset ID, fee-payer checks, simulate.
• Point machine-readable errors at @x402/avm VerifyErrorReason.
• Settlement: confirm transaction committed before reporting success.

Docs-only.

Made with Cursor

[cb-heimdall]

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