feat: align whole-grain definitions across diet sources

[koen-vg]

Unify the whole-grain definition on the food-level taxonomy and the GBD exposure basis:

• Add a maize-whole food (whole_grains, FBS item 2514, USDA FDC 169697) fed from the maize crop, mirroring the wheat and rice whole/refined pairs, so maize-staple regions have a whole-grain food to carry GBD's whole-grain exposure.
• Refit diet.fbs.whole_grain_shares jointly against GBD per-country whole-grain exposure (population-weighted bounded least squares, countries whose always-whole coarse-grain base exceeds the GBD target excluded as uninformative): flour-wholemeal 0.07, rice-brown 0.01, maize-whole 0.46.
• Re-split GDD-IA's total cereal energy by each country's FBS cereal composition instead of GDD's own whole/processed split, which counts decorticated millet and sorghum as processed and starved Sahel staples in GDD mode (NER whole grains 27 -> 249 g/day). Cereal masses are now kcal-derived at model-basis group densities and the kcal-target cereal columns carry the aligned split.
• Disable piecewise food utility explicitly in the feed, food_waste and food_demand calibration configs; their enforced-baseline validation solves reject it, and base configs like central.yaml enable it.
• Refresh all three calibration artefact sets (default, gdd-ia, gbd-anchored). Stability L1 costs land at or below their previous centres and all cereal food_demand multipliers stay at 1.0 with zero slack, confirming the realignment introduces no supply/demand mismatch.