Add FeatureMLP benchmark adapter

[nipunbatra]

Summary

• register the tested nilmtk-contrib FeatureMLP model with the standard benchmark protocol
• pin benchmark installs and published image builds to reviewed contrib commit 9ce85b3
• update registry and container contracts without adding an unmeasured leaderboard row

Verification

• 168 tests passed with 76.77% coverage
• ruff check src tests
• uv lock --check
• wheel and sdist build
• generated leaderboard JSON and CSV match committed artifacts
• pinned FeatureMLP class resolves and constructs on CPU
• corrected T1 REDD FeatureMLP dry-run succeeds

The real REDD/A100 result will be published separately with its immutable source and image provenance.