[META] Production-ready client-side data model + internal HTTP API

🐛 Empty slot placeholder not rendering for content that has been published Active and 📌 [PP-1] Rename PropSourceComponent.field_data to propSources Postponed are in.

🐛 Component transforms need to be per sourceType, not per component prop Active and 📌 Define API URI naming conventions Active landed.

Oops. #3519992 is part of 📌 SdcController cleanup tasks Active .

Removed #3499352 from this meta and into 📌 Only run XbTwigExtension inside XBPreviewRenderer Active per #3499352-15: No XB-targeted HTML comments on the live site: only run XbTwigExtension inside XBPreviewRenderer → .

Added two missing things for the client-side data model:

1. Missing "node type" in docs/data-model.md's 3.4 UI Data Model: communicating a `component tree` to the front end: a node type for exposed ContentTemplate slots: #3519352-32: Content templates, part 3b: store exposed slot subtrees on individual entities →
2. Missing "node type" in docs/data-model.md's 3.4 UI Data Model: communicating a `component tree` to the front end: a node type for overrides/personalization variants

#20.2 is overkill, based on a call I just had with @effulgentsia and @penyaskito — thanks @effulgentsia for raising, as you heard, I'd already been thinking in the same direction 😊