v10.23: language-aware candidate-goal promotion playground case

Summary

• add one new governed non-runtime playground case for language-aware candidate-goal promotion
• index the new case in the playground README
• keep the playground outside the active 18-file runtime install payload

Verification

• reinstalled the active runtime rules into ~/.claude/rules
• verified 18/18 source/runtime parity plus source/destination body sufficiency
• ran git diff --check clean