[4b] Flip the host write path on by default #454

Part of Phase 4 (#427) of the data layer plan.

Flip the default; keep the old path for one release.

• Rolling back means a release that flips the default again.

Done when: the release ships on the host path.

Depends on:

• 4a ([4a] Wire the WebView to the host behind a feature flag #453), with its edit-latency check passed.
• If that check fails, the issue it adds for an optimistic layer.
• The plan's decision on reading every book at session open, recorded from the 2e checkpoint (Read the whole text in the host with a form-in-text index #474).