fix(ci): enforce non-zero coverage gate threshold (closes #589)

[s2x]

Description

Closes #589

The CI coverage gate was present in name only: the threshold was 0.0 (trivially green at the measured ~82.17% coverage) and the same check ran twice per matrix leg. This PR makes the gate effective.

Changes

• Threshold 0.0 → 80.0, defined once in composer.json (coverage:check) — 80% leaves ~2.2pp headroom against the observed 82.17%, while the measured 0.04pp per-leg spread makes flaky failures impossible
• Removed the duplicate invocation — .github/workflows/tests.yaml now calls composer coverage:check only; the verbatim php bin/check-coverage.php var/coverage.xml 0.0 duplicate is gone, so the threshold lives in exactly one place
• Gate runs once, not nine times — the Check coverage threshold step is restricted via if: to the lowest supported matrix leg (PHP 8.2 / Symfony 6.4.*); per-leg coverage differs by ~0.04pp so the remaining eight gates were wasted signal. Per-leg coverage reports are still uploaded as artifacts (if: always() preserved)
• Benchmark comment clarified — the ci aggregation job's "advisory initially" comment now states the advisory mode is intentional (runner timing is too variable to gate merges on)
• Regression test hardened — CoverageCiGateTest now asserts: single gate invocation tied to the correct step, the 8.2/6.4.* leg condition, no direct check-coverage.php call in the workflow, upload step keeps if: always(), and a non-zero threshold defined in composer.json
• Docs — docs/workflow.md step 7 and CONTRIBUTING.md now state the 80% floor and where it is defined, so contributors learn about it before CI tells them

Not implemented (explicitly optional in the issue): the coverage ratchet. 80.0 is the recommended safe first step; a ratchet can follow separately.

Changelog

Added under [Unreleased] → Fixed: make the CI coverage gate effective (threshold 80% defined once in composer.json, duplicate removed, single-leg gate).

Code Review

• Passed subagent code review
• All review comments addressed

Gate failure demonstration

The gate was demonstrated to fail correctly by a temporary commit (now removed) that excluded three covered test directories (tests/Command, tests/Phar, tests/Supervisor) from the PHPUnit suite. Coverage dropped from ~82% to 68.21%, and the gate failed on the single 8.2 / 6.4 leg:

Coverage: 68.21% (6213/9108 statements). Threshold: 80.00%. FAILED

Failing run (8.2/6.4 leg): https://github.com/crazy-goat/workerman-bundle/actions/runs/30379541493/job/90343929802

All other 8 matrix legs stayed green (the gate only runs on the designated leg), Lint and Benchmark passed, and the coverage report artifact still uploaded (if: always()). The temporary commit was removed via force-push; the branch now contains only the three implementation commits.

Note for reviewer

#597 also touches .github/workflows/tests.yaml (per its author's comment on #589) — whichever lands second should rebase.