ci: make Accept gate fail when any required job fails

[prestwich]

Summary

The Accept job in ci.yml was a single-step run: "true" that depended on all the real CI jobs. With default GitHub Actions semantics, when any job in needs: fails, dependent jobs are skipped, not failed.

GitHub branch protection treats a skipped required status check as non-blocking, so the required Accept check never reported failure even when test/clippy failed. PR #576 hit exactly this — automerge fired with failing CI because Accept was reported as skipped rather than failure.

This PR runs Accept unconditionally (if: always()) and explicitly fails when any dep is not success, turning it into a real failing required check that branch protection blocks on.

Test plan

• Open this PR; confirm Accept runs and reports success when all jobs pass.
• Push a commit that breaks a test on a draft PR; confirm Accept reports failure (not skipped) and that branch protection blocks merge.

🤖 Generated with Claude Code

[DaniPopes]

can we do this?
https://github.com/alloy-rs/core/blob/2ebfa5b502e535d9864497579ebac7eeae30e27d/.github/workflows/ci.yml#L232-L253

[codspeed-hq]

Merging this PR will degrade performance by 10.41%

❌ 2 regressed benchmarks
✅ 384 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	base_convert/128/128/10/be	13.6 ms	15.2 ms	-10.41%
❌	most_significant_bits/4096/4096	26 µs	29 µs	-10.32%

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_{Comparing prestwich/fix-accept-gate (ed9be8c) with main (c910174)}

[prestwich]

🫡