Document one-step distilled specialists in MNEL architecture

[epi13]

Summary

• add ADR 0003 establishing one-step distilled specialists as a MNEL learned-provider pattern;
• distinguish one learned forward pass from snapshot construction, deterministic checks, verification, and escalation;
• define MNEL/MNCS/Forge/Fabric/Harness ownership and evidence-based admission requirements;
• update the roadmap to call for teacher, baseline, control, calibration, abstention, and transfer measurements.

Validation

• Documentation-only change.
• Compared main with docs/one-step-distilled-specialists: five expected documentation files, one commit, no unrelated changes.
• GitHub Actions CI should run the existing Python and Rust verification suites.