Add Lua test coverage reporting (luacov + CI integration)

[levleontiev]

Summary

Add Lua source coverage measurement via luacov so we can track which branches in src/fairvisor/*.lua are exercised by unit and integration tests.

Motivation

Currently CI runs busted spec/unit/ spec/integration/ (497 tests) but does not measure coverage. Python e2e pytest-cov measures test infrastructure — not useful. The only meaningful coverage metric for fairvisor is Lua coverage from busted.

Acceptance Criteria

• luacov installed in docker/Dockerfile.test
• bin/ci/run_lua_tests.sh runs busted --coverage spec/unit/ spec/integration/ and generates luacov.report.out
• CI job uploads coverage artifact
• (stretch) Coverage badge in README.md via codecov.io or shields.io
• Existing CI job coverage-report updated to use Lua coverage instead of Python e2e coverage (which measures test infrastructure, not business logic)

Implementation Notes

• busted --coverage flag requires luarocks install luacov
• For badge: luacov-cobertura converts luacov.stats.out → Cobertura XML → upload to codecov.io → badge URL
• Alternative lighter approach: parse luacov.report.out in CI and post summary as step output

Out of scope

• Python e2e coverage (removed from scope — those files are tests themselves)
• Per-file coverage gating (just reporting for now)