Fix TOU derivation to use MC-based seasonal rates directly

[sherryzuo]

What

The TOU derivation script (utils/pre/compute_tou.py) currently computes demand-weighted average MCs per season, then rescales them so the load-weighted mean equals a base_rate from a calibrated flat tariff (from runs 1 and 2). This discards the MC absolute level and anchors to a reference tariff, creating unnecessary upstream dependencies:

• run-9 → needs run-1 to get nyseg_flat_calibrated.json as the base rate anchor
• run-10 → needs run-2 to get nyseg_flat_supply_calibrated.json as the base rate anchor

Why

Since CAIRO's precalc recalibrates the absolute level anyway, rescaling to a reference tariff base rate is unnecessary. Eliminating this dependency lets TOU tariffs be generated entirely in all-pre, decouples runs 9 and 10 from runs 1 and 2, and makes the rate derivation more semantically correct (rates reflect actual MC; precalc calibrates revenue neutrality).

How

1. utils/pre/compute_tou.py — Remove base_rate parameter from compute_seasonal_base_rates; return raw demand-weighted average MCs directly (no rescaling).
2. utils/pre/derive_seasonal_tou.py — Remove tou_base_rate parameter from derive_seasonal_tou(); update the call to compute_seasonal_base_rates; remove --tou-base-rate CLI arg; keep --reference-tariff but use it only for fixedchargefirstmeter (not base rate); remove the tou_base_rate is None validation guard.
3. rate_design/hp_rates/Justfile — Add create-tou-tariffs recipe that runs after create-flat-tariffs and update all-pre accordingly; simplify run-9 and run-10 to just call run-scenario.
4. utils/Justfile — Update create-seasonal-tou recipe description to reflect the new role of --reference-tariff.
5. tests/test_derive_seasonal_tou.py — Remove tou_base_rate=0.1 from derive_seasonal_tou() call.
6. context/code/marginal_costs/compute_tou_from_marginal_costs.md — Update to reflect that rates are output directly from MC without rescaling.

Deliverables

• PR merging the changes above
• Runs 9 and 10 pass without relying on run-1/run-2 outputs
• just check and just test pass

[linear]

RDP-168