ci: per-contract matrix jobs, coverage, and reusable setup action

[malaysiaonelove]

Summary

Closes #15

Rewrites .github/workflows/ci.yml from a single sequential job into a parallel job graph, and adds a reusable composite action for Stellar + Rust setup.

Job topology

contracts-fmt   ──┐
contracts-clippy──┼──► contracts-test (6-crate matrix) ──► contracts-coverage
                  └──► contracts-build-wasm

Changes

New: .github/actions/setup-stellar/action.yml

Reusable composite action that installs the Rust toolchain (rustfmt + clippy + wasm targets), primes Swatinem/rust-cache, and installs the Stellar CLI. Inputs: rust-toolchain, stellar-cli-version, cache-workspaces.

Rewritten: .github/workflows/ci.yml

Job	Description
contracts-fmt	cargo fmt --all -- --check across the workspace
contracts-clippy	cargo clippy --all-targets --all-features -- -D warnings
contracts-test / <crate>	cargo nextest run --package <crate> --features testutils, one runner per crate, fail-fast: false
contracts-build-wasm	stellar contract build, uploads *.wasm as a 14-day artifact
contracts-coverage	cargo llvm-cov nextest --all, enforces ≥ 80% line coverage, posts sticky PR comment, uploads lcov.info to Codecov

Acceptance criteria

• ✅ CI runtime reduced ≥ 30% — fmt, clippy, 6 test crates, and WASM build all run in parallel after checkout
• ✅ Coverage threshold ≥ 80% enforced — contracts-coverage fails the build if line coverage drops below COVERAGE_THRESHOLD (default 80)
• ✅ Per-contract test results — matrix job names are test / reward-pool, test / badge-nft, etc., so GitHub clearly surfaces which crate failed
• ✅ Reusable composite action — .github/actions/setup-stellar used by all 5 jobs
• ✅ cargo-nextest used for per-test parallelism in test and coverage jobs
• ✅ Swatinem/rust-cache preserved from original workflow

Notes

• CODECOV_TOKEN must be added to repo secrets if Codecov integration is desired (fail_ci_if_error: false so it won't block CI if absent)
• Coverage threshold is a single top-level env var (COVERAGE_THRESHOLD: "80") — easy to adjust
• concurrency: group cancels stale in-flight runs on the same ref