Matt Bierner 245265d15e Mark most Event properties as readonly ...

We want to prevent mistaken changes that do something like this:

```ts
foo.onEvent = () => { ... };
```

When they almost always mean:

```ts
foo.onEvent(() => { ... })
```

2025-10-09 13:59:00 -07:00

..

common

Mark most Event properties as readonly

2025-10-09 13:59:00 -07:00

test/common

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