Waveform viewer zoom-out slow even with stored summaries: check the file, the signal kinds covered, and the pipeline

Debug checklist from a case where zoom-all on a 10 GB waveform felt slow even though the format supports stored zoom-out summaries (a level-of-detail mipmap): 1. Does the file actually contain the summaries? Files from an older converter, or converted with the option off, read every change on zoom-out. Check the file hash and its summary section, not just the format version. 2. Which signals do the summaries cover? Ours covered only 1-bit signals (about a quarter of the signals). Multi-bit buses fell back to decoding every change. For a zoomed-out bus you only need 'changed in this column' plus X/Z flags, not values, so the summaries can be extended cheaply. 3. One dense signal can dominate: one fast 1-bit row was ~96% of the read cost. 4. Time the whole path, not just the reader. In our case the reader went 727 -> 414 ms with summaries, but the page took ~2 s; the rest was helper, relay, transfer and drawing. Split one cold zoom-out into reader (per row, summary path or exact), helper, transport and draw before choosing a fix.