feat(specs): add Radix exact scheme

[AVaunt-consulting]

Description

Adds the exact scheme specification for Radix DLT (Babylon networks): specs/schemes/exact/scheme_exact_radix.md, plus the Radix entry in the Critical Validation Requirements summary in scheme_exact.md.

Per the spec-first process, this PR contains the spec only; the SDK implementation will follow once the spec is approved.

Highlights:

• Two settlement modes: sponsored (recommended; the client signs a payment subintent, the facilitator wraps it in a root transaction and pays fees - clients need no XRD) and non-sponsored (the client signs a complete notarized transaction).
• VERIFY_PARENT binding restricts which facilitator can consume a sponsored subintent, preventing front-running of the bearer payload.
• Facilitator verification mandates strict instruction-sequence validation, fee-payer isolation, timestamp-based expiry mapped from maxTimeoutSeconds, replay protection, and preview (CommitSuccess with exact balance deltas) before submission.

CAIP alignment

Network identifiers use the CAIP-2 radix namespace (radix:mainnet, radix:stokenet). We have submitted the corresponding namespace registration - CAIP-2, CAIP-10, and CAIP-19 profiles for Radix - to the ChainAgnostic namespaces registry, currently in review: ChainAgnostic/namespaces#198

Tests

Docs-only change (no SDK code). Mermaid sequence diagrams were validated with mermaid-cli; example bech32m addresses are checksum-valid.

Checklist

• I have formatted and linted my code
• All new and existing tests pass (docs-only change)
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)

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