Fix use of stale props in Fabric events (#26408)

## Summary

We had to revert the last React sync to React Native because we saw
issues with Responder events using stale event handlers instead of
recent versions.

I reviewed the merged PRs and realized the problem was in the refactor I
did in #26321. In that PR, we moved `currentProps` from `canonical`,
which is a singleton referenced by all versions of the same fiber, to
the fiber itself. This is causing the staleness we observed in events.

This PR does a partial revert of the refactor in #26321, bringing back
the `canonical` object but moving `publicInstance` to one of its fields,
instead of being the `canonical` object itself.

## How did you test this change?

Existing unit tests continue working (I didn't manage to get a repro
using the test renderer).
I manually tested this change in Meta infra and saw the problem was
fixed.