docs/language: a page for every keyword and attribute the vocabulary has

Keywords: module uses friend export internal numbers unsafe mut port bind action
reducer registry of as from def open alias component prop view (structure),
shows lasts then loads (scenes), system (ecs), dispatch (control), true false
null (operators). Attributes: @Ref @OneOf @Range @Unit @Asset @Color, @Node /
@Clip / @Material, @Tint @Derived, @Text / @Multiline, @Key, @AppendOnly /
@ByKey, @PerMap / @Chunked, @frame @max, @owns / @creates / @releases,
@deterministic @alloc_ok. Each is in tools/docgen/inventory.json; every fence
that is not marked skip parses (ludicc --fmt). annot-clearcolor's token loses its
quotes, which no reader strips.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/language/structure/kw-reducer.md

@@ -0,0 +1,20 @@
+---
+id: kw-reducer
+name: reducer
+category: structure
+kind: keyword
+tokens: reducer
+sig: reducer State on Action(st: mut State, reads..., a: Action) { ... }
+tip: Applies an action to one state; it may read others.
+order: 62
+---
+
+A <code>reducer</code> writes exactly one state when its action is dispatched: the first parameter is that state, <code>mut</code>, the last is the action, and any states between are read-only. A change that must touch several states in a set order is a chain: a reducer dispatches the next action.
+
+```ludic
+state Pos { x: int = 0 }
+action Move { dx: int = 0 }
+reducer Pos on Move(p: mut Pos, a: Move) {
+  p.x = p.x + a.dx
+}
+```