[codex] Implement E-09 interpreter sandbox

[PilHliP211]

Summary

• Add the E-09 acorn-based simplified-JS interpreter with AST sandbox validation, friendly player errors, and step-mode execution.
• Add a temporary /__dev/interpreter harness backed by Flag Hunter dev content for the human-testable increment.
• Add unit and invariant coverage for allowed calls, sandbox rejections, friendly errors, loop limits, and step execution.
• Mark all E-09 story ticket acceptance checklists complete.

Validation

• npm run lint
• npm run typecheck
• npm run test
• npm run format:check
• npm run build
• curl -I http://127.0.0.1:5173/codequest-platform/__dev/interpreter returned 200 OK during the dev-server check

Notes

• Validation was run with NVM Node v22.22.2, matching the repo engine requirement.