Code coverage threshold

[gloskull]

Motivation

• Require a minimum line coverage for PRs to prevent regressions in test coverage across the workspace.
• Add the tooling and caching needed to produce and check coverage reliably in GitHub Actions.
• Document the CI coverage gate and the local reproduction steps in the repository README.

Description

• Add COVERAGE_THRESHOLD: "80" and an Enforce code coverage threshold step to .github/workflows/rust.yml that runs cargo llvm-cov --fail-under-lines "$COVERAGE_THRESHOLD" to fail CI below 80% line coverage.
• Configure the workflow to install llvm-tools-preview, target wasm32-unknown-unknown, cache cargo outputs with Swatinem/rust-cache@v2, and install cargo-llvm-cov via taiki-e/install-action@cargo-llvm-cov for coverage instrumentation.
• Update README.md with a new “Continuous Integration” section describing the coverage gate and how to run the equivalent local command using rustup component add llvm-tools-preview and cargo llvm-cov.
• No source code logic was modified; changes are limited to CI workflow and documentation.

Testing

• Ran git diff --check and committed the workflow and README changes successfully.
• Verified cargo build --verbose completed successfully (build OK).
• Ran the test suite with cargo test --verbose; the run produced 131 passed; 17 failed and overall test result: FAILED, with failures localized to pool_manager tests that panic because Soroban storage is accessed outside of a contract context (needs env.as_contract()), so the coverage gate itself was not the cause of the failures.
• Ran repository checks including python scripts/storage-layout-check.py (passed) and parsed the workflow YAML with Ruby (passed).

Closes #86