Pin Rust toolchain to pre-1.82 for soroban-sdk 21 wasm compatibility

[priscaenoch]

Summary

Pin Rust toolchain to pre-1.82 for soroban-sdk 21 wasm compatibility

What was solved

#13 — Pin the Rust toolchain with a rust-toolchain.toml

Add a root rust-toolchain.toml pinning the Rust channel (pre-1.82, e.g. 1.81.0) with targets = ["wasm32-unknown-unknown"] and components rustfmt/clippy, then reference the pinned toolchain in README build notes and the CI workflow so local and CI builds match and avoid the reference-types/multivalue wasm issue with soroban-sdk 21.

Addressed:

• Changed: rust-toolchain.toml, rust-toolchain.toml, README.md, .github/workflows/ci.yml
• Add new rust-toolchain.toml at repository root pinning the channel and declaring targets = ["wasm32-unknown-unknown"] and components rustfmt and clippy
• Pin a channel below Rust 1.82 (e.g. 1.81.0) since 1.82+ emits reference-types/multivalue that soroban-sdk 21's VM rejects
• Reference the pinned toolchain in README build notes

Changes

• README.md (modify)
• .github/workflows/ci.yml (modify)
• rust-toolchain.toml (create)

Approach

1. Create rust-toolchain.toml at repo root pinning channel = "1.81.0" with targets = ["wasm32-unknown-unknown"] and components = ["rustfmt", "clippy"]
2. Update README.md build notes to mention the pinned toolchain (rustup auto-selects it) and keep the existing wasm-opt step wording intact
3. Update .github/workflows/ci.yml to ensure the pinned toolchain is used (e.g. rustup show / rustup target add wasm32-unknown-unknown) without altering unrelated jobs
4. Verify no other files, tests, snapshots, or workflows are modified

Issues

Closes #13