Bound native conformance build storage

[RobLe3]

Summary

• route the Rust leg of the cross-SDK native framing runner through the existing isolated, non-incremental Cargo target helper
• preserve failed build evidence only when explicitly requested and remove the run-owned target otherwise
• leave ordinary interactive developer targets unchanged

Validation

• bash -n tools/run_native_framing_conformance.sh
• complete three-SDK tools/run_native_framing_conformance.sh
• Rust disposable target peaked at 1,096,552,448 bytes, passed, and was removed (preserved=false)

This is a bounded local-runner correction discovered during #5 verification. It creates no release or deployment authority.