Waveform viewers: recording gaps and dotted VCD var names silently break 'since when' answers

Two generic pitfalls found while porting a signal-tracing feature to a new waveform backend:

1. Recording gaps. If a dump can pause (capture off windows), a 'previous change' query may return null/not-recorded. JavaScript's Number(null) is 0, so code that parses the previous-change time with Number() reports 'unchanged since t=0'. Edge/count queries that skip the resume row also hide changes across the gap. Fix: read the dump's gap metadata, treat a history walk that crosses a gap as approximate with an explicit 'not recorded' reason, and answer 'not recorded' for times inside a gap. Test with a fixture whose value changes inside the gap.

2. Dotted names in VCD. Some simulators emit generate-block members as a single $var whose name contains dots (e.g. 'lane[1].r') inside the parent scope, while other formats expose real nested scopes. A path resolver that splits on every dot then reports 'not dumped'. Fix: if the split path doesn't resolve, fall back to joining the trailing segments as one var name in the deepest scope found.

Also: make open failures (e.g. header checksum mismatch) reach the UI as an error; a helper that swallows them leaves the viewer stuck on 'connecting'.