refactor(config): hardcode the Cornerstone model schema, delete use_cornerstone_2026_model_schema

[briantobin-99]

cc:
Closes:

What changed? Why?

use_cornerstone_2026_model_schema is deleted. Every config set it True — the Cornerstone 2026 model schema is the only supported schema — so all code paths now assume the flag's True side unconditionally. 40 files, net −719 lines.

Model pipeline.

• transform/eeio/derived.py: the legacy wrappers collapse to their cornerstone implementations; derive_B_usa_via_vnorm and scale_abq_via_summary.py (orphaned legacy-B helpers) are deleted, along with the unused inflate_A_matrix/inflate_B_matrix/inflate_q_or_y helpers in inflation_helpers_ceda.py.
• transform/allocation/derived.py: map_fbs_sectors_to_model_schema always maps into Cornerstone_2025 (the electricity-disaggregation retargeting hook is unchanged); the E collapse reindexes to the Cornerstone columns directly, and the legacy BEA→CEDA groupby collapse is gone.

Allocator chain. The MECS mapping selectors, get_allocation_sectors(), and load_bea_use_table() are pinned to the Cornerstone frame — the side that generated the pinned *_CEDA allocation FBA parquets on GCS, which carry Cornerstone-frame activity codes (33131B, 335220, disaggregated 562*). The use-table alignment helpers for CEDA-vocabulary sectors are unchanged.

Diagnostics. EF diagnostics always align old (CEDA v7) vs new (Cornerstone) EF vectors, always emit comparison_type and the sector_mapping_notes tab.

Guards and shims. require_cornerstone_config (publish) and the _force_cornerstone_model_schema context manager (nowcast) are deleted with their call sites — they only asserted or forced the flag.

Config. The field is removed from USAConfig and the key stripped from all config YAMLs. Behavior-neutral: every YAML set it True, and USAConfig's strict validation requires removing the key in the same change.

Testing

• Unit suite, black, ruff, and mypy green.
• The v0.3 snapshot integration tests (test_usa.py, 9 tests) pass against the pinned snapshots.
• The allocation FBA builders were re-run end-to-end and reproduce the pinned GCS parquets bit-identically: BEA_CEDA_foam_allocation_2023, BEA_CEDA_refrigeration_allocation_2023, and EIA_MECS_Energy_Allocation_CEDA_2018 (2,835 rows; rebuilt under usa_ghg_data_year: 2023, the vintage the pin was generated with — the EPA allocation tables used by the MECS builders do not yet support 2024).

Snapshot integration run for this branch: https://github.com/cornerstone-data/bedrock/actions/runs/30674351415