feat: add exact payment scheme spec for XRPL

[aristotle-satoshi]

Add specification for the exact payment scheme on XRP Ledger, supporting both native XRP and IOU (issued currency) payments.

Description

This PR adds the exact payment scheme specification for the XRP Ledger (XRPL), following the pattern established by existing chain-specific specs (EVM, SVM, Sui, Aptos).

Key features:

• Support for native XRP payments (amounts in drops)
• Support for IOU (Issued Currency) payments with issuer validation
• CAIP-2 network identifiers: xrpl:0 (Mainnet), xrpl:1 (Testnet), xrpl:2 (Devnet)
• Invoice binding via Memos or InvoiceID field
• Comprehensive facilitator verification rules
• Security considerations for replay protection and partial payment prevention

Tests

This is a specification document only - no code changes.

• Spec structure matches existing scheme specs (scheme_exact_evm.md, etc.)
• Uses MUST/SHOULD/MAY language per specs/CONTRIBUTING.md guidelines
• Includes JSON examples for PaymentRequirements and PaymentPayload
• Documents Security Considerations section

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)

[cb-heimdall]

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[vercel]

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[phdargen]

Hi @aristotle-satoshi, thanks a lot for putting this together! However, we already have an ongoing PR for the exact XRPL scheme, see #777.

Closing yours to keep discussion about this topic in one place. Please review #777 and check how it differs from your proposal, your feedback would be much appreciated!