ci: run octocov checks in parallel

[kitsuyui]

Summary

• Run independent lint, test, and build checks in .github/workflows/octocov.yml with GitHub Actions step-level parallel execution.
• Keep octocov reporting sequential after the coverage-producing test step completes.

Validation

• Parsed .github/workflows/octocov.yml as YAML.
• bun run lint
• bun run test

Notes

• Local actionlint 1.7.12 does not recognize the new parallel step syntax yet.

[github-actions]

🎉 Happy commit!

• dogfooding: PR ci: run octocov checks in parallel #852 always celebrates.
• dogfooding: 2c0803904da9f4978fa964f66fdd93445f7ae547 always celebrates.

[github-actions]

Code Metrics Report

	main (b2e78fd)	#852 (efc2c0b)	+/-
Coverage	99.2%	99.2%	0.0%
Code to Test Ratio	1:1.3	1:1.3	0.0
Test Execution Time	1s	2s	+1s

Details

  |                     | main (b2e78fd) | #852 (efc2c0b) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          99.2% |          99.2% | 0.0% |
  |   Files             |              5 |              5 |    0 |
  |   Lines             |            134 |            134 |    0 |
  |   Covered           |            133 |            133 |    0 |
  | Code to Test Ratio  |          1:1.3 |          1:1.3 |  0.0 |
  |   Code              |            714 |            714 |    0 |
  |   Test              |            966 |            966 |    0 |
- | Test Execution Time |             1s |             2s |  +1s |

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Reported by octocov