Harden Soroban executor

[dkgoutham]

Brings the Stellar/Soroban Executor to parity with the EVM/Sui/Aptos/SVM
references.

Commits

1. fix(executor): pass native token address via constructor — removes the
   hardcoded testnet native SAC (was wrong on mainnet); the token address is now
   injected at deploy and read from instance storage.
2. feat(executor)!: accept signed quote as opaque bytes (breaking) — the
   quote is taken as opaque Bytes, its 68-byte header parsed by offset, and the
   full quote (header + body + signature) emitted verbatim so relayers can verify
   off-chain. SignedQuote removed; quoter_address is a 20-byte EVM identity.
3. test(executor): cover wire-format parsing and event fidelity — 9 tests
   covering the happy path, byte-for-byte event preservation of a full 165-byte
   EQ01 quote.