docs: automated documentation pipeline (per-symbol source → pages)

Replace the hardcoded landing page and minimal reference with a generated
documentation site driven by a single source of truth.

- docs/language/**: one file per symbol (93 keywords/types/builtins/namespace
  methods/operators/annotations), each with front-matter (id, kind, tokens,
  sig, tip) + description + a ```ludic example. Seeded by exploding the former
  inline SECTIONS list; these files are now the source of truth.
- docs/site/: site.json (editable hero/features/showcase/messaging, not
  hardcoded) + snippets/*.ludic (real programs shown on the landing page).
- tools/docgen/gen.py: generates index.html, api.html, ludic-highlight.js and
  symbols.json. The highlighter's symbol tables, hover tips and jump anchors
  are GENERATED from the per-symbol files — add a symbol and it is recognized,
  tipped and linked in every snippet automatically. Python stdlib only.
- tools/docgen/check.py: verifies the pages contract + that no snippet token
  links to a missing reference anchor.
- .forgejo/workflows/docs.yml: rebuilds and publishes to the pages branch on
  every push to main touching the docs sources.

Consumes the new Screen.*/Color.*/named-arg API and the 221-color palette.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/scenes/_section.md

@@ -0,0 +1,7 @@
+---
+id: scenes
+title: Scenes & layers
+order: 3
+---
+
+One active scene at a time, each grouping handlers into layers.

docs/language/scenes/kw-enter.md

@@ -0,0 +1,12 @@
+---
+id: kw-enter
+name: enter
+category: scenes
+kind: keyword
+tokens: enter
+sig: on enter { … }
+tip: The scene-entry hook.
+order: 3
+---
+
+The scene-entry hook.

docs/language/scenes/kw-exit.md

@@ -0,0 +1,12 @@
+---
+id: kw-exit
+name: exit
+category: scenes
+kind: keyword
+tokens: exit
+sig: on exit { … }
+tip: The scene-exit hook.
+order: 4
+---
+
+The scene-exit hook.

docs/language/scenes/kw-layer.md

@@ -0,0 +1,12 @@
+---
+id: kw-layer
+name: layer
+category: scenes
+kind: keyword
+tokens: layer
+sig: layer Name { handlers }
+tip: A group of handlers inside a scene; layers render in declaration order.
+order: 1
+---
+
+A group of handlers inside a scene; layers render in declaration order.

docs/language/scenes/kw-on.md

@@ -0,0 +1,12 @@
+---
+id: kw-on
+name: on enter / on exit
+category: scenes
+kind: keyword
+tokens: on
+sig: on enter { … }   on exit { … }
+tip: Hooks that fire when a scene becomes active or is left.
+order: 2
+---
+
+Hooks that fire when a scene becomes active or is left.

docs/language/scenes/kw-scene.md

@@ -0,0 +1,16 @@
+---
+id: kw-scene
+name: scene
+category: scenes
+kind: keyword
+tokens: scene
+sig: scene Name [start] { on enter{} on exit{} layer … }
+tip: A mutually-exclusive game state.
+order: 0
+---
+
+A mutually-exclusive game state. Mark one <code>start</code>.
+
+```ludic
+scene Title start { on enter { … } layer Main { … } }
+```