feat(ci): add Lua coverage reporting and badge pipeline

[levleontiev]

Summary

• replace misleading Python E2E coverage path with Lua coverage via luacov
• generate coverage summary and badge JSON payloads from CI
• publish badge payload through GitHub Pages and wire README to the Pages-backed shield
• add a non-generated Lua coverage regression gate so autogenerated files do not hide coverage drops

Verification

• docker run --rm --user "$(id -u):$(id -g)" -v "$PWD:/work" -w /work fairvisor-test-ci bin/ci/run_lua_coverage.sh
• bin/ci/build_lua_coverage_badge.sh
• bash -n bin/ci/run_lua_coverage.sh bin/ci/build_lua_coverage_badge.sh
• LUA_COVERAGE_MIN_NON_GENERATED=82.29 bin/ci/build_lua_coverage_badge.sh fails as expected

Notes

• Current local metrics: 98.47% total, 82.28% non-generated
• Badge/gate use the non-generated metric because src/fairvisor/generated/bot_automaton.lua would otherwise dominate the denominator
• Draft PR for now; not calling review until the full required test/CI path is confirmed

Refs #24