docs: simplify happens-before explanation

[jadecubes]

Summary

• replace the wide per-queue matrix with one compact release/acquire chain
• keep the key SPSC insight that availability checking and synchronization share the same load
• map mutex, SPSC, and MPMC publication briefly while leaving the full proof in docs/design.md

Verification

• cmake --build build-check --parallel 4
• ctest --test-dir build-check --output-on-failure --no-tests=error (65/65 passed)
• pandoc --from=gfm --to=html README.md
• git diff --check origin/main..HEAD

Review

Independent review found no Critical, Important, or Minor issues.