docs(mobile/4): document the screen lifecycle events

[sylvesterdamgaard]

Docs for the events added in NativePHP/mobile-air#248, as requested there. That PR is approved but not yet merged — happy to hold this until it lands.

What's here

A new Observing the lifecycle from outside section on Lifecycle Hooks, covering ScreenMounted / ScreenResumed / ScreenUnmounted: when each fires, what they carry, and how to listen.

It leads with why they exist rather than what they are, because that's the part a reader can't infer. The hooks are the app's to override, so a base class's mount() is the wrong home for anything cross-cutting — a screen defining its own mount() silently replaces it, and the observer goes quiet on exactly the screens with the most logic in them. That failure mode is invisible until you go looking for missing data, so it seemed worth stating outright.

Two behaviours are documented that a reader would otherwise have to find in the source:

• a listener that throws is caught and logged rather than propagated, so "my listener isn't running" has a place to look
• restoring a preloaded stack after a hot reload stays quiet, since those screens are restored rather than navigated to

Also added a short pointer from Events, which until now only covered component-targeted native events — a reader looking for "how do I observe navigation" would land there first and find nothing.

Notes

• v4 only. The events are on the v4 line, and v3 has no digging-deeper section.
• No new page, so no index or ordering changes.
• Table separator style and the relative-anchor convention match the surrounding pages.
• Section placed before Related so the hook reference stays contiguous.

Verified the uri claim against the router rather than assuming it: unmountComponent() runs before array_pop() at all seven call sites, so ScreenUnmounted reports the unmounting screen's own uri, not the one underneath it.