Fix empty block running average calculation ## Summary The register report's running average calculation was incorrect for empty blocks (months with no transactions). Consecutive empty blocks all showed the same average value instead of each dividing by an increasing block count. ## Bug When viewing a quarterly subscription with monthly blocks and `--begin`/`--end` (to force empty month generation), the averages were wrong: | Month | Running Total | Before (bug) | After (fix) | |-------|--------------|--------------|-------------| | Jan | 120.00 | 120.00 | 120.00 | | Feb | 120.00 | 60.00 | 60.00 | | Mar | 120.00 | **60.00** | **40.00** | | Apr | 255.00 | 63.75 | 63.75 | | May | 255.00 | 51.00 | 51.00 | | Jun | 255.00 | **51.00** | **42.50** | Feb and Mar had the same average because `blockNum` wasn't incremented. ## Root cause In `centjes/src/Centjes/Report/Register.hs`, both `generateEmptyBlocks` (multi-currency and single-currency variants) and the trailing empty block generation in `goBlocks` passed the same `blockNum` to every empty block without incrementing it in the recursive call. ## Fix Increment `blockNum` by 1 in each recursive call within `generateEmptyBlocks` and in the trailing empty block generation within `goBlocks`. Fixed in all 4 locations (multi-currency and single-currency paths, both mid-register and trailing empty blocks). ## Regression test Added `quarterly-subscription-monthly-average-with-begin` golden test: a quarterly electricity subscription viewed with monthly blocks and `--begin`/`--end`, which generates empty months between transactions and exercises the average calculation across gaps. ## Things to manually check - The 4 existing golden tests that were updated (`convert-with-date-range`, `convert-year-filter`, `end-date-price-change`, `convert-with-end-date`) - confirm the new averages look correct for their scenarios.