feat(schemes): add escrow scheme for usage-based payments

[gigabit-eth]

Description

Adds the escrow scheme specification for session-based, usage-metered payments within x402.

This scheme complements the existing exact scheme by enabling deferred settlement patterns where:

• Payment amounts are determined after resource consumption
• Funds are locked in escrow and released based on actual usage
• Batch settlement reduces on-chain transactions from per-request to per-session

Use cases: Metered API access, LLM inference, compute services, multi-call agent workflows

Files added:

• specs/schemes/escrow/scheme_escrow.md - Core scheme specification
• specs/schemes/escrow/scheme_escrow_evm.md - EVM implementation (Base Sepolia)
• specs/schemes/escrow/scheme_escrow_svm.md - Solana session layer w/ cross-chain relay (experimental)

Reference implementation: cart.fun contracts

Tests

This PR adds specification documentation only—no code changes requiring tests.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• Issue Created: x402 Escrow Scheme for Pre-funded, Usage-Based Payments #839

[cb-heimdall]

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[cb-heimdall]

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