wiki: add node-definitions doc for three-checkbox composition model

New page covering the geometry/renderer/system trio that registry-driven
kinds opt into. Documents:

- The three optional fields on NodeDefinition and when each applies
- Generic <GeometrySystem> + <ParametricNodeRenderer> runtime
- GeometryContext shape (resolve / children / siblings / parent)
- Combination matrix for shelf / spawn / zone / door / window / GLB items
- Migration recipe from custom renderer+system files to def.geometry
- Rules around purity, dispose-on-rebuild, register-once

renderers.md and systems.md gain "prefer registry-driven" banners and
link out to the new page. Architecture README adds the page to the
index so review-architecture skill picks it up.

The pattern was validated by the shelf spike: inline-JSX geometry was
visibly laggy on parametric edits; moving to a per-kind system reading
dirtyNodes (mirroring door/wall/item) restored smoothness. The three-
checkbox model generalises that win so most future kinds need only a
pure geometry function.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

wiki/architecture/README.md

@@ -9,6 +9,7 @@ Canonical rules for code that touches `packages/core`, `packages/viewer`, `packa
 | [layers](layers.md) | Three.js layer constants, ownership, and rendering separation |
 | [systems](systems.md) | Core and viewer systems architecture |
 | [renderers](renderers.md) | Node renderer pattern in `packages/viewer` |
+| [node-definitions](node-definitions.md) | Three-checkbox composition model for registry-driven kinds (`geometry` / `renderer` / `system`) |
 | [tools](tools.md) | Editor tools structure in `apps/editor` |
 | [viewer-isolation](viewer-isolation.md) | Keeping `@pascal-app/viewer` editor-agnostic |
 | [selection-managers](selection-managers.md) | Two-layer selection (viewer + editor), events, outliner |