Pin the Rust toolchain with a rust-toolchain.toml

[priscaenoch]

Problem

The repository has no rust-toolchain.toml, so builds use whatever Rust version is installed. This is unusually risky here because the README documents a hard toolchain constraint: these contracts pin soroban-sdk 21, whose VM rejects the WebAssembly reference-types and multivalue features, while Rust 1.82 and later emit those features into every wasm it builds — including the standard library — and -C target-feature=-reference-types does not reliably remove them. The project works around this with a wasm-opt lowering step in build-and-deploy.sh.

Without a pinned toolchain, contributors and CI can silently produce a wasm that fails deployment for reasons unrelated to their change, and the required wasm32-unknown-unknown target is not declared anywhere.

What needs to be done

• Add a rust-toolchain.toml at the repository root pinning the channel and declaring targets = ["wasm32-unknown-unknown"] and the components used (rustfmt, clippy).
• Verify the pinned toolchain builds both crates and that the wasm-opt lowering step still produces a deployable artifact.
• Reference the pinned toolchain in the README build notes and in the CI workflow.

Files

• new rust-toolchain.toml
• README.md
• .github/workflows/ci.yml

Acceptance deliverables

• A fresh clone builds with the pinned toolchain and no manual rustup steps.
• CI uses the same toolchain as local development.

Tests to pass

• cargo build --release --target wasm32-unknown-unknown succeeds for both crates on the pinned toolchain.
• The resulting wasm deploys successfully after the documented wasm-opt step.

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #13)
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