chore(ci,repo): pin Rust 1.85.0 and wasm32v1-none target

[K1NGD4VID]

Summary

Aligns CI with the README's mandated toolchain and Stellar's expected wasm target — a combined landing of the three interdependent tickets that could not be shipped separately without breaking CI.

• Adds rust-toolchain.toml at repo root pinning channel = 1.85.0 + targets = [wasm32v1-none] + components = [clippy, rustfmt] so contributors get the same toolchain locally with zero rustup setup.
• Updates .github/workflows/ci.yml:
  • dtolnay/rust-toolchain@stable → @master with explicit toolchain: 1.85.0 (matches README:127)
  • targets: wasm32-unknown-unknown → wasm32v1-none (matches Stellar CLI's expected target, README:134)
  • Release build step retargeted to wasm32v1-none so CI artifacts are bit-for-bit comparable to stellar contract build output
• Points README quick-start at the new toolchain file — the rustup toolchain install steps stay as an install fallback.

Why bundled

#285 alone (adding rust-toolchain.toml with wasm32v1-none) would break CI because the workflow still installs wasm32-unknown-unknown and builds against it. #283 and #284 alone would leave the client-side gap (contributors still get whatever rustup default is set to). All three must land together — hence the combined PR after #333 was closed as premature.

Test plan

• CI build-and-test job goes green on this PR
• cargo build --workspace --release --target wasm32v1-none succeeds on the runner
• cargo test --workspace still passes (host target unchanged)
• cargo fmt --all --check and cargo clippy --all-targets --all-features -- -D warnings still pass

Closes #283
Closes #284
Closes #285