fix(mpc): cost every challenger with Core's own model

.changeset/shadow-costed-by-core.md

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+---
+"ftw": patch
+---
+
+The Python shadow's verdict is now Core's price for both plans. Core walks
+the challenger's own action sequence through its forward pass and costs it
+with the same arithmetic the DP uses on itself, instead of reading the total
+the challenger arrived with — and it does the same for its own plan, so the
+subtraction stays symmetric if the DP's bookkeeping ever moves. The
+difference keeps the key `python_minus_core_ore_terminal_corrected`.
+
+What the challenger reported is kept beside the verdict rather than inside
+it: `self_reported_ore` is the cost it claimed for its own plan, and
+`self_reported_objective_ore` is the value it actually minimized —
+scenario-weighted and CVaR-shaped, so not comparable with any cost. On the
+owner's box those two differ by 450 öre on a single plan.
+
+A plan Core cannot cost honestly — wrong length, non-finite output, a
+battery driven outside its operating band or past its power limits — is
+recorded as `evaluation_refused_reason` with no difference number at all.
+Core costing its own plan differently from what it reported is a bug in
+Core, and now says so with a warning and `active_evaluation_drift_ore`.
+
+The replay bench measures the same way, prints the challenger's own figure
+in its own column, and takes `FTW_MPC_BENCH_CVAR_WEIGHT`,
+`FTW_MPC_BENCH_CVAR_ALPHA` and `FTW_MPC_BENCH_MIP_GAP`. Those knobs matter:
+a snapshot records the planning inputs but not the site's solver settings,
+so replaying one with the bench's defaults asks the challenger a different
+question than the box asked — worth more than 130 öre per plan on a summer
+day with PV uncertainty, which is larger than the gap being measured.
+
+A second bench asks what the price twin is worth. It re-solves each snapshot
+three ways — the confidences the box used, those same forecast slots
+flattened to the horizon mean, and the forecast slots deleted — and reports
+the only number that reaches hardware: the first slot's battery power. On
+every snapshot recorded so far the three agree exactly, and they keep
+agreeing until the known window falls under four hours.