bi-measure-builder

# BI Measure Builder Most wrong BI numbers are not syntax errors. They are the right formula evaluated in the wrong context. Work context-first, and prove every measure against numbers computed independently. ## Workflow 1. **Pin down the model before writing anything.** Get (or state as an explicit assumption): the fact table and its grain (one row per order line? per snapshot?), dimension tables, relationship keys and direction, and the date table. Ask only for what changes the formula; otherwise state the assumption and proceed. Why: the same YoY formula is right on a star schema and wrong on a flat table with a text date. 2. **Name the platform and version features.** DAX: classic vs calendar-based time intelligence (preview), marked date table or not. Tableau: live vs extract, context filters in use. Looker: dialect (window-function support, `period_over_period` support). 3. **State the evaluation context in one or two sentences** for a typical visual cell *and* for the grand total. Example: "Row = Month; filters = Region slicer; total = all months in the slicer range, recomputed, not summed." Why: this is where most bugs live, and saying it forces the check. 4. **Write the calculation** from the matching pattern in the reference file. Use variables (DAX `VAR`) for readability and to evaluate each sub-expression once. 5. **Explain it** line by line in terms of context: what each filter argument adds, removes, or overrides; where context transition happens; which Table...