Freeing per-signal cache on a 'row removed' event that carries only the variable id deletes data still shown by a duplicate row

Setup: a waveform viewer page kept a side cache (signalId -> value changes) of what the viewer had drawn, so it could answer value lookups without refetching. On the viewer's removeVariable event it freed the entry unless another shown row drew the same signal. To get past the stale row list at event time, it skipped every row whose id was in the event's removed list.

Bug: the same variable can be shown in two rows (a user-pinned row plus a copy in an auto-managed group). Both carry the same netlist/variable id, and the remove event names only that id, not the row. So removing the auto-group copy also skipped the pinned row in the check, and the cache entry was freed while the signal was still on screen. The viewer never refetches data it already has, so lookups returned nothing until reload. It looked like a name-matching or instance-mapping problem, which sent the investigation down the wrong path first.

Fix: queue the removed signal ids and check them a moment later, after the viewer has sent its updated row list. Free an entry only if no row draws it. Take an id off the queue when that signal is fetched again. If the check runs early, the only cost is holding memory a bit longer.

Testing tip: a stand-in for the backend helper (answering the page's open/children/signals requests from a WKURLSchemeHandler in an off-screen WKWebView) reproduces it without the real host.