fix(capacity): clear harness quality gate findings

[ttbombadil]

Summary

• Remove Sonar findings in the mixed-capacity harness while preserving existing measurement behavior.
• Parse Docker metadata only as strings, decode WebSocket payload types explicitly, simplify the Linux memory key regex, and extract option validation.
• Invoke the pinned Arduino CLI from the installer's fixed /usr/local/bin path.

Verification

• Capacity unit tests: 26 passed.
• npm run check and ESLint on changed files passed.
• Full npm run sonar passed, including its coverage run.
• No production load run was performed; target Linux capacity remains unmeasured.