[codex] Implement E-08 syntax editor

[PilHliP211]

What changed

Implemented E-08 Syntax Editor (Phase 3):

• Added a lazy-loaded Monaco syntax editor with controlled value handling, read-only mode, Run button wiring, and restricted completion items from content-pack availableFunctions.
• Added the Phase 3 EditorToggle with Code and read-only Blocks tabs, ARIA tab behavior, keyboard navigation, and fallback after repeated failed attempts.
• Added temporary /__dev/syntax-editor harness backed by JSON content so the epic has a human-testable npm run dev increment before the real lesson runner exists.
• Added syntax unlock persistence through progressStore using the existing storage helper instead of direct component-level storage calls.
• Marked the E-08 story Done When checkboxes complete.

Impact

Phase 3 now has a developer-visible syntax editor surface and automated coverage for the core editor/toggle/unlock behaviors. The harness is documented as temporary and should be folded into the real LessonScreen path when E-11/E-14 wire the product flow.

Validation

• npm run lint
• npx tsc --noEmit
• npm run test — 160 tests passed
• npm run format:check
• npm run build
• npm audit --omit=dev — 0 vulnerabilities
• Browser smoke: Chrome/Playwright loaded /codequest-platform/__dev/syntax-editor and found .monaco-editor

Notes

npm run build passes with Vite's expected large chunk warning for Monaco. The editor is lazy-loaded into a separate SyntaxEditor chunk instead of the main bundle.