docs: document how to verify a deployed contract matches a local build

[DevSolex]

Summary

Closes #144

Adds a Verifying a deployment section to the README that gives any integrator the complete steps to confirm a live contract was compiled from this source tree.

What was added

A new section between ## Deploying to testnet and ## Project structure covering three steps:

1. Local build — cargo build --release --target wasm32v1-none with the pinned toolchain, then sha256sum the artifact.
2. On-chain hash — two methods: stellar contract inspect --id <CONTRACT_ID> --network <NETWORK> (reads the wasm_hash field from the contract instance) or stellar contract fetch + sha256sum (downloads and hashes the bytecode directly).
3. Compare — matching hashes confirm the deployment.

The section also documents the reproducibility caveat honestly: Linux hosts produce consistent hashes with the pinned toolchain; macOS and Windows hosts may see a different hash even with identical source due to platform-specific LLVM output.

Acceptance criteria met

• README documents how to fetch a deployed contract and compare it with a local build
• Commands are complete enough to follow without prior knowledge
• Reproducibility caveat is stated honestly

Verification

cargo fmt --check          ✓
cargo clippy --all-targets -- -D warnings  ✓
cargo test                 ✓  (105 tests passed)

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