fix(lifecycle): restore backtest readback metadata

[Pigbibi]

Scope

BacktestResult.to_dict() persists cost_inputs and validation_identity, but PerformanceStore._backtest_from_dict() discarded both. This made the new exact run readback unable to verify the cost assumptions saved by the SOXL/TQQQ synthetic replay.

Restore those fields on readback. Keep missing legacy fields at the existing empty defaults and reject explicitly malformed metadata. Save paths and latest baseline selection are unchanged. A stored identity remains metadata and does not authorize promotion; the existing promotion gates still require complete orchestrator, folds, OOS and source evidence.

Evidence

• uv run --frozen python -m unittest tests.test_lifecycle_performance_store -q: 18 tests OK.
• uv run --frozen --with pytest python -m pytest tests/test_lifecycle_performance_store.py tests/test_strategy_performance_export.py -q: 27 passed.
• uv run --frozen --with pytest python -m pytest tests/test_backtest_orchestrator.py -q: 27 passed, 11 subtests passed.
• Targeted Ruff and git diff --check: passed.
• UES synthetic replay against this local QPK source: 13 passed. This is an import-path preview, not an updated UES lock.

No real cloud store, market data, accounts, orders, deployment or production configuration were accessed. The repository's pre-existing lock mismatch remains unchanged, so the local QPK checks used --frozen.