feat: isolate Solana wallet authorization signing

[TFT444]

Closes #33

Summary

• add a SolGuard Ed25519 issuer and wallet-side public-key verifier
• bind authorization to agent, wallet, recipient, USDC mint, atomic amount, devnet network, request digest, nonce, and expiry
• inspect serialized transaction bytes independently before signing
• atomically consume each authorization exactly once before the private-key backend is invoked
• load a disposable offline Ed25519 wallet seed only from an external environment source
• emit a sanitized signing receipt after a real cryptographic signature is produced
• provide injected inspector/backend boundaries for offline, adversarial, concurrency, and failure tests

Security properties

• missing, expired, replayed, malformed, incorrectly signed, or mutated authorizations fail closed
• wrong wallet, network, mint, recipient, amount, nonce, expiry, agent, or request digest fails before signing
• mainnet is disabled
• only one concurrent caller can consume an authorization
• store and signer failures burn or reject the authorization safely
• secret material and serialized transaction bytes are absent from receipts
• SolGuard authorization and wallet signatures remain separate proofs

Verification

• 487 tests passed
• 100% statement and branch coverage
• strict mypy passed for 41 source files
• Ruff lint and format checks passed
• staged Gitleaks scan found no leaks

Review structure

This stacked PR is based on feature/32-autonomous-attack-runner / PR #37. Review commit f7db88e for the focused #33 delta.

The canonical transaction codec is explicitly an offline fixture. It cryptographically signs bytes but does not claim to create a Solana wire transaction or devnet settlement; that evidence belongs to #34.