Double invoke effects in a subtree wrapped with StrictMode

[Andarist]

Summary

Currently, strict effects don't behave as expected when StrictMode isn't placed at the root of the app. Wrapping only a part of the app is a valid use case described in the docs: https://react.dev/reference/react/StrictMode#enabling-strict-mode-for-a-part-of-the-app

How did you test this change?

I added tests in the repo to verify that it works 😉

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I expect some subtleties around this so this might not be a fully correct PR (yet). With some guidance I could improve it. I'm especially not sure if the code paths related to Offscree/Visibility are changed correctly. If you could suggest some extra test cases for them, I'd appreciate it

[react-sizebot]

Comparing: ff62833...89afbaa

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	515.43 kB	515.43 kB	=	92.03 kB	92.03 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	558.46 kB	558.46 kB	=	99.25 kB	99.25 kB
facebook-www/ReactDOM-prod.classic.js	=	636.83 kB	636.83 kB	=	111.91 kB	111.91 kB
facebook-www/ReactDOM-prod.modern.js	=	627.15 kB	627.15 kB	=	110.33 kB	110.33 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 5af49d5

[Andarist]

1. The problem is that in this scenario only the root gets PlacementDev flag because reconcileChildren routes to mountChildFibers for all of the descendant's fibers.
2. That in turn is a reconciler function created with shouldTrackSideEffects === false
3. So .subtreeFlags can't be relied upon because, well, those flags are not set on those descendant fibers so they are also not propagated onto their ancestors' .subtreeFlags

That led me to propagate the treeFlags through the callers' chain here