Fast zoom-to-fit first draw for huge waveform dumps: bucket-start samples plus density bands, never sample periodic signals

Problem: a viewer that opens a multi-GB dump at zoom-to-fit must walk each visible signal's whole history before the first draw (tens of seconds cold).

Pattern that keeps the first draw honest: - Split the visible time range into B pixel-ish buckets (B ~ 1000). Fetch the exact value at each bucket start with one batched reader call: one traversal handle per signal, sorted seeks. Cost is about one reader open floor plus B seeks. Measure the per-seek cost with a probe before building. - For busy signals, don't draw samples at all. Use a cached per-signal change count to estimate changes per bucket. If it's at least about 1, draw a 'busy' band, which is what the exact render shows at that zoom anyway. - Never draw periodic signals (clocks) from samples: they alias into a fake slow waveform. Use a clock descriptor (period/phase) or the density band. - Flag the reply approx:true, draw those rows dimmed or hatched, and replace them when the exact background walk finishes. - Document what the coarse view misses: changes that return to the same value within a bucket (glitches, pulses), x/z intervals shorter than a bucket, and edges placed only to bucket resolution. Medium-density signals (0.05-1 change per bucket) are the worst case: they look calmer than they are. - Some formats rebuild derived signals from the start of the dump, so seeking is not cheap for them. Keep those rows on the slow exact path. - Cheap alternative: open on a short range (a few % of the dump, around a marker) and run zoom-to-fit in the background. Most commercial viewers do this.