Detect startValue/endValue in .speedscope.json files

When omitting startValue/endValue, Speedscope hangs the browser when using current-day timestamps:

```JSON
{
"$schema": "https://www.speedscope.app/file-format-schema.json",
"version": "0.0.1",
"profiles": [
{
"events": [
{
"type": "O",
"frame": 0,
"at": 1592385255
},
{
"type": "C",
"frame": 0,
"at": 1592385255
}
],
"type": "evented",
"unit": "seconds"
}
],
"shared": {
"frames": [
{
"name": "foo"
}
]
}
}
```

If not present, SpeedScope should detect the range of events automatically.

Use the minimal .speedscope.json example in the issue as a reproduction, then trace profile loading and event-range handling in the TypeScript application. Verify the behavior with current-day timestamps and omitted startValue/endValue. Done means the event range is detected automatically and the browser no longer hangs.

Written by the indexing model from the issue text.