docs(ui): add design document for the Forge UI system

.cspell/project-words.txt

@@ -1,14 +1,20 @@
 aabb
 aabbs
+actioned
+activatable
+affordances
 afterrun
+agnostically
 Aoboshi
 artboard
+atlased
 attw
 autofetch
 backquote
 Batchable
 Baumgarte
 Beziers
+blit
 blits
 cachable
 Catmull
@@ -20,11 +26,14 @@ deregistering
 desaturates
 despawn
 devcontainers
+diegetic
 eamodio
 Erdokovy
 esbenp
+evented
 Fira
 Flaticon
+flexbox
 fphysics
 fract
 frontmatter
@@ -39,9 +48,13 @@ highp
 hitscan
 hotspot
 hoverable
+imgui
+implementability
 impluse
 Infima
 inigo
+interactable
+interactables
 Kenney
 kenneynl
 keyup
@@ -52,9 +65,12 @@ matterjs
 mediump
 Menlo
 Mertens
+minimaps
 mousedown
 mousemove
 mouseup
+msdf
+multiplicatively
 Narkowicz
 ndot
 Nisbet
@@ -65,13 +81,17 @@ pingpong
 Pixabay
 prebuild
 prestart
+quadtree
 quilez
 rasterizer
+raycaster
 readback
+rects
 refac
 reinhard
 Renderable
 renderables
+reparenting
 requizits
 resettables
 respawns
@@ -103,6 +123,7 @@ thresholding
 thumbstick
 tileable
 tintable
+tweening
 unblurred
 undersamples
 unmarks
@@ -113,6 +134,8 @@ unvalidated
 updatables
 upsamples
 uvec
+uxml
+viewports
 Vleugels
 WASD
 webgl

AGENTS.md

@@ -61,8 +61,16 @@ Forge is a browser-based, code-only game engine built with TypeScript. It provid
 /documentation-site        # Docusaurus documentation
 /scripts                   # Build and utility scripts
 /assets                    # Static assets (images, etc.)
+/design                    # Design docs / RFCs for not-yet-built subsystems
 ```
 
+`/design` holds forward-looking design documents for subsystems that don't exist
+yet (architecture, decision logs, phased backlogs). They are proposals, not
+descriptions of current behavior - never treat a `/design` document as a source
+of truth about what the engine does today, and don't cite one in user-facing
+documentation. Once a design ships, `documentation-site/docs/docs` becomes the
+authority and the design document stays as historical rationale.
+
 ### Module Exports
 
 The package exports are modular. Each subsystem has its own export path: