Publish PE-native target diagnostics sidecar

[PavelMakarchuk]

Closes #133.

Summary:

• publish a first-class PE-native per-target diagnostics sidecar as pe_native_target_diagnostics.json
• reuse existing PE-native target delta payloads for checkpoint evidence and native-audit backfills
• add dashboard/API-friendly per-target aliases for aggregates, absolute/relative errors, deltas, loss contribution, family, and support flags
• wire the sidecar through stage contracts, stage refs, validation evidence, stage status, and experiment artifact paths

Tests:

• uv run --extra dev --python 3.13 python -m pytest -q tests/pipelines/test_pe_native_scores.py tests/pipelines/test_pe_us_data_rebuild_checkpoint.py tests/pipelines/test_backfill_pe_native_audit.py
• uv run --extra dev --python 3.13 python -m pytest -q tests/pipelines/test_stage_run.py tests/pipelines/test_stage_contracts.py tests/pipelines/test_experiments.py
• uv run --extra dev --python 3.13 python -m ruff check changed files

Note: full-repo Ruff still has unrelated pre-existing failures outside this branch.