send less data to build system web interface clients #24651
Description
Split from #24588.
Currently, when using zig build --webui, connected clients receive data about the whole compilation over the WebSocket connection. In particular, when --time-report is passed, clients are sent -- and will store -- the full time report for every step in the compilation, which may involve a large amount of data.
Instead, to save memory and WebSocket bandwidth, large chunks of data should probably only be sent on-demand. For instance, clients should by default only be told whether or not time reports currently exist for each step; the actual time report should be sent to clients only when they request it.
This also applies to the fuzzer: clients should only receive updates about fuzz testing status when the user opens the corresponding fuzz test in the UI.