AIFP-3 vNext SDK: one passport across all chains

[coinsecuritiescompany]

Updates @aifinpay/agent for the finalized AIFP-3 vNext global passport model.

Implemented:

• one immutable agent_id / permanent AIFP number with multiple verified wallet bindings
• local Ed25519 holder-key generation; private key stays with the agent
• global passport issuance helpers
• holder challenge signing
• wallet-binding challenge/confirmation helpers
• issuer verification against an independently pinned AiFinPay public key
• protected SHA-256 payload hash + Ed25519 signature verification
• fail closed on integrity_state=failed and blocked/revoked wallets
• exact-network wallet selection; no silent cross-chain fallback
• backward-compatible resolver fetch injection preserved
• published package version bumped to 2.0.0-rc.2

Verification:

• GitHub Actions CI run #271: SUCCESS
• @aifinpay/agent TypeScript build + Vitest + npm pack passed
• MCP build against the agent in this commit passed
• MCP package, wallet package, Python SDK and examples gates passed

Backend counterpart: AiFinPay/aifinpay-web PR #33.
Smart-contract changes intentionally follow after this off-chain identity interface.