Render children in fibers

So far doesn't achieve what we want because a lot of children get rendered in theme templates when #theme is set, not via Renderer directly.

This is working as demonstrated by the profiling/testing in 📌 Try to replace path alias preloading with lazy generation Active .

Postponing this on 📌 Pass RenderContext around in the Renderer Active which changes the same code. I think that overall we probably don't want to do this, and instead should continue with adding #lazy_builder/#create_placeholder to more things. However 📌 Try to replace path alias preloading with lazy generation Active provides an easy way to test the effectiveness of this, so it might be worth one more try once things are a bit further along.

Actually postponed on 📌 Deprecate RendererInterface::render()'s sole $is_root_call parameter Needs work .

Rebased on top of that to check something.

This mostly helps when rendering tabs, but because we don't have admin performance tests, it's not picked up in test coverage yet.

I rebased the branch on top of 📌 Deprecate RendererInterface::render()'s sole $is_root_call parameter Needs work so that it's easier to test alongside other issues later.

Updated the issue summary.