[Infra] Reproducible WASM build + size budget

[Gbangbolaoluwagbemiga]

Why this matters

Reviewers and auditors need to confirm that the deployed WASM matches the source. The build must be
reproducible and the artifact published with a checksum. A size budget prevents accidental bloat
(WASM size drives deploy cost on Soroban).

What needs to be done

• Pin the build via rust-toolchain.toml (already present — verify it covers the wasm target)
• Produce the optimized .wasm (stellar contract build / soroban contract optimize)
• In release.yml, attach the optimized .wasm + its sha256sum to each GitHub Release
• Add a CI gate failing if the optimized WASM exceeds a documented byte budget
• Document the reproducible build steps in the README/CONTRIBUTING

Acceptance criteria

• Two clean builds produce byte-identical WASM
• Release contains .wasm + .sha256
• Oversized WASM fails CI

Relevant files

.github/workflows/release.yml, rust-toolchain.toml, Cargo.toml