fix(runnative): propagate isolated session roots

[joeykchen]

Summary

• run native Engine children through the shared interpreted-session command path
• replace stale root variables with validated project, asset, and session roots
• remove duplicate Engine argument rewriting

Verification

• go test ./...
• go vet ./...
• go test -race ./cmd/spx/internal/command ./internal/interpruntime
• spx runnative -headless on .tmp/Coding-Course-49 ran until interrupted

[fennoai]

Reviewed the complete fixed diff for the pack-mode runtime handoff and interpreted-session root propagation. The new command preparation keeps SessionDir authoritative for --path, replaces stale root environment variables, and preserves the existing asset/session setup contract; the affected callers and platform-specific test helper are consistent with that behavior.

Validation completed with go test ./cmd/spx/internal/command ./internal/interpruntime, go vet ./cmd/spx/internal/command ./internal/interpruntime, and go test -race ./cmd/spx/internal/command ./internal/interpruntime. No actionable correctness, security, performance, or maintainability findings meet the review threshold.