Implement token adapter pattern for multi-standard token support

[Smartdevs17]

Description

Currently only native Soroban tokens are supported. Implement a token adapter pattern to support ERC-20, native Stellar, and wrapped tokens through a unified interface.

Acceptance Criteria

• Token adapter trait with standard interface
• Native Soroban token adapter
• ERC-20 adapter for EVM tokens
• Wrapped token adapter (Stellar-native wrapped)
• Adapter registry for token type discovery
• Formal verification of adapter safety

Technical Scope

• stellar-lend/contracts/lending/src/token_adapter.rs
• stellar-lend/contracts/lending/src/token_adapter_erc20.rs
• stellar-lend/contracts/lending/src/token_adapter_native.rs
• stellar-lend/contracts/lending/src/token_adapter_wrapped.rs
• stellar-lend/contracts/lending/src/token_adapter_verify.rs
• Edge: reentrancy via adapter callback, decimal mismatch, fee-on-transfer tokens