ci: enforce a 95% per-crate coverage gate in contracts.yml (#1120)

[jjb9707]

Summary

Adds a per-crate line-coverage gate to .github/workflows/contracts.yml: the build fails when any crate under onchain/contracts/ drops below 95% line coverage.

Changes

.github/workflows/contracts.yml

• Adds llvm-tools-preview to the Rust toolchain components (required by cargo-llvm-cov for instrumentation)
• Installs cargo-llvm-cov via taiki-e/install-action (pre-built binary, faster than cargo install)
• Adds a Per-crate coverage gate (fail under 95% lines) step: the workspace is instrumented once with cargo llvm-cov --workspace --json, then line counts are attributed back to each crate and every crate is checked against the threshold individually
• Prints a per-crate table sorted worst-first to both the job log and the job summary, and on failure emits a ::error:: annotation naming how many crates are below the bar
• The threshold lives in the COVERAGE_THRESHOLD env var on that step

docs/ci.md — documents the gate, the exact commands to reproduce the CI numbers locally, the extra prerequisites, and a troubleshooting entry.

Why not --fail-under-lines on its own

cargo llvm-cov --workspace --fail-under-lines 95 only enforces a workspace-wide average, so a single uncovered crate can hide behind well-covered ones. This issue asks for a per-crate bar, so instrumentation is collected once (one test run, keeping the gate cheap) and then attributed per crate. Files under a tests/ or src/tests/ directory are excluded, so test code cannot inflate a crate's own score.

Testing

• The gate's aggregation and pass/fail logic were exercised end-to-end against cargo llvm-cov JSON-export fixtures covering: src/tests/ exclusion, crate-root tests/ exclusion, multi-file crates aggregated per crate rather than judged per file, a crate exactly on the threshold (95.00 -> pass), and a crate below it (-> below 95% row plus non-zero exit).
• CI-config change only; no contract code is modified.

Note on the current red check

The contracts job stops earlier, at the pre-existing cargo fmt --all -- --check step. That step checks the whole repository, and reports the same 143 violations across 39 files on main as it does on this branch, so the failure is not introduced here and the coverage-gate step is never reached. Once formatting is resolved on main, this gate will run.

Closes #1120