nx_meal_cost.nx

nx_meal_cost.nx -- LIB: the PANTRY x PRICE bridge. nx_pantry knows what a unit already HAS; nx_price knows what things COST and where. Nothing joined them, so no surface could answer the question a meal planner exists to answer: "given what is already in my kitchen, what does cooking THIS actually cost me today, and what is cheap right now?" TWO HONESTY INVARIANTS, both gate-enforced, because both failures make food look cheaper than it is: 1. AN UNPRICED INGREDIENT IS NEVER WORTH ZERO. An item we hold no observation for is counted in its own bucket (MC_UNPRICED) and NEVER folded into the total. Summing only what we happen to know the price of produces a confident, precise, WRONG number -- the most expensive kind. 2. A CHEAP BASKET AT A STORE THAT STOCKS ALMOST NOTHING IS NOT A CHEAP BASKET. pr_basket deliberately SKIPS items a store does not carry, so a shop stocking 1 of 5 ingredients posts the lowest total and would win "cheapest store" outright. mc_best_store therefore requires FULL coverage before a store may compete at all. "On sale" is MEASURED, never claimed: it means this store's recorded price sits at least `min_delta_cents` below the area MEDIAN for that item (nx_price's own competitiveness frame). The threshold is a PARAMETER, never a literal in here (rule 11) -- and a non-positive threshold FAILS CLOSED rather than marking the whole shop on sale. Prices stay what nx_price says they are: dated observations with a source, surfaced with mc_price_asof / mc_price_source so a page can never imply live pricing it does not have. Integer cents throughout, no float. license_tier: ORIGINAL No hw writes (Rule 26).