Add Hive scheme specification for x402

[feruzm]

Description

Add Hive blockchain support to the x402 exact scheme specification, including the current v1 flow and the implemented v2 wire format.

Hive is a delegated-proof-of-stake blockchain with zero-fee transactions and a native stablecoin (HBD, pegged to $1 USD). This makes it well suited for x402 micropayments: no gas costs, ~3 second finality, and no token approval steps.

This PR adds:

• specs/schemes/exact/scheme_exact_hive.md - full Hive exact scheme specification covering payload structure, verification logic (secp256k1 signature recovery against on-chain active key authorities), settlement logic (direct transaction broadcast), and v2 wire-format documentation
• Updates to the generic upto scheme text to clarify versioned payment amount field semantics
• A Hive upto spec draft, scoped as v1 only and explicitly not claiming current implementation support

A TypeScript reference implementation for Hive exact is available at @hiveio/x402 (source), and a public facilitator is live at https://x402.ecency.com.

Tests

This PR is specification-only (no code changes to the x402 codebase).

Hive exact behavior is exercised in the reference implementation at ecency/hive-x402, including:

• End-to-end 402 payment flow
• Signature verification
• Replay rejection / nonce handling
• Error handling
• Express, Next.js, and Hono middleware flows

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• Changelog fragment for user-facing changes (docs-only changes can skip)

[cb-heimdall]

🟡 Heimdall Review Status

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[vercel]

@feruzm is attempting to deploy a commit to the Coinbase Team on Vercel.

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[feruzm]

Hi @Must-be-Ash! Could you guide me on what are the next steps to get this reviewed, please?