Refactor MC generators into utils/pre/marginal_costs/ package with shared utilities

[sherryzuo]

What

Refactor the three marginal cost generator scripts into a dedicated utils/pre/marginal_costs/ package with a shared common.py module, replacing the current flat layout under utils/pre/.

Current layout:

• utils/pre/generate_utility_supply_mc.py (energy LBMP + capacity ICAP)
• utils/pre/generate_utility_tx_dx_mc.py (distribution + sub-transmission via PoP)
• utils/pre/generate_bulk_tx_mc.py (bulk transmission via SCR)

Proposed layout:

utils/pre/marginal_costs/
├── __init__.py
├── common.py          # shared utilities (see below)
├── supply.py          # energy LBMP + capacity ICAP (from generate_utility_supply_mc.py)
├── distribution.py    # distribution + sub-tx via PoP (from generate_utility_tx_dx_mc.py)
└── bulk_transmission.py  # bulk Tx via SCR (from generate_bulk_tx_mc.py)

Why

The three generators share significant code that is currently handled by cross-imports:

• build_cairo_8760_timestamps() — supply MC defines it, bulk Tx imports it
• load_utility_load_profile() + normalize_load_to_cairo_8760() — tx_dx MC defines them, supply MC and bulk Tx import them
• load_zone_mapping() + get_utility_mapping() — supply MC defines them, bulk Tx imports them
• S3 save/output patterns are similar across all three

This creates a fragile import graph. A shared common.py would make the dependency clearer and reduce coupling.

How

1. Create utils/pre/marginal_costs/ package with __init__.py.
2. Extract shared functions into common.py: build_cairo_8760_timestamps, load_utility_load_profile, normalize_load_to_cairo_8760, load_zone_mapping, get_utility_mapping.
3. Move each generator into the package, updating imports to use common.
4. Update all consumers: tests (tests/test_marginal_cost_allocation.py, tests/test_bulk_tx_mc.py), Justfiles (rate_design/hp_rates/ny/Justfile, rate_design/hp_rates/Justfile), and run_scenario.py.
5. Verify just check and just test pass.

Deliverables

• utils/pre/marginal_costs/ package with common.py and three generator modules
• All existing tests pass with updated imports
• Justfile recipes updated to point to new module paths
• No remaining cross-imports between generator modules (all shared code in common.py)

[linear]

RDP-143