Adding a second waveform format to a windowed viewer: fill the existing store, don't add a new answer path

When a waveform viewer already has a windowed protocol (window/valueAt/edge/search/count) served by one in-memory per-signal store, add a new dump format as a data source that fills that store (or a temporary range of it) from the format's blocks. Then normalisation, bucket counts, edge kinds and search stay the same code, and parity tests only have to cover the fill. Check first whether the store's value encoding already matches the format: a 2-plane 4-state layout (val, xz) with 0=(0,0) 1=(1,0) x=(0,1) z=(1,1) is common, so the fill becomes a plain copy. Gotchas: decimated tiles that shade by exact per-bucket change counts can't be served from a LOD pyramid whose counts are capped at 0/1/many. A ranged fill has to include the last change before t0, or the 'drop repeats of the value in effect' normalisation diverges at window edges. Also: store a normalised change count per signal in the container, so that 'load small signals whole' works at open with no walk.