Pin toolchain and document SDK/CLI versions

[iexwr]

Context

Cargo.toml pins soroban-sdk = 22.0.0, but the README does not state the required CLI/SDK versions, leading to mismatched local builds.

Acceptance criteria

• Add a "Requirements" line to README.md with the exact soroban-sdk (22.x) and a known-good stellar CLI version.
• Add rust-toolchain.toml pinning a stable Rust + wasm32-unknown-unknown.

Technical notes

• Reproducible-build hygiene; no contract logic change.

Testing

• cargo build --target wasm32-unknown-unknown --release succeeds on the pinned toolchain.