chore(repo): DX cleanup — categorise examples, text-diffable golden, build/ output (#27 #28 #30)

Repository-cleanup / DX pass folding three tracker items into one coherent
change, verified green end to end (`bin/x test` 49/0, `bin/x selfhost-test`
29/0, `bin/x test-tools` 29/0).

#28 — curate & categorise examples/
- 42 flat entries regrouped into intent-revealing subdirs: games/, rendering/,
  ecs/, events/, networking/, lang/, library/ (was lib/).
- chronorift dir-vs-file duplication resolved: the entry file and its import
  modules now live together under games/chronorift(.ludic).
- Every path reference updated repo-wide (test runner, editor-tool drivers,
  docs/site, design docs).
- New examples/README.md indexes the whole set with run commands.
- Showcase examples without a self-asserting entry (hello, events, net_rt) now
  get a compile-only rot guard in `bin/x test`, so nothing here rots silently.

#30 — text-diffable golden baseline
- The 4 binary selfhost/golden/*.ppm blobs are replaced by a single
  selfhost/golden/renders.sha256 manifest (SHA-256 per render). Hashes are
  byte-identical to the old PPMs, so the baseline is unchanged — only its form.
- game_case now compares framebuffer hashes; a regression shows as a changed
  hex line in review, not "binary files differ".
- New `bin/x golden` regenerates the manifest deliberately (review with
  `git diff selfhost/golden/renders.sha256`).

#27 — PPM & asset handling
- Headless renders now write build/out.ppm, never the repo root; `x app`,
  `x clean`, messaging and .gitignore updated to match. Nothing is written to
  the working root any more.
- Redundant local Kenney .zip archives removed (the art ships extracted;
  .gitignore already excludes *.zip). CC0 License.txt files retained.

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

examples/lang/scenes.ludic

@@ -0,0 +1,48 @@
+# scenes — one active scene at a time, each grouping handlers into layers behind
+# an implicit active-scene register (see LANGUAGE.md §"Scenes & layers").
+#
+# Running it (feed a few keystrokes so the loop ticks) prints:
+#   1000 1 101 102 2 3 900 201 900 202 900
+#     1000     Boot (a global handler) runs once at Start
+#     1        Title is `start`; its `on enter` fires at boot
+#     101      frame 1 Update: only Title.Main.Tick runs (Play is not active)
+#     102      frame 2 Update: Tick reaches 2 -> `become Play`…
+#     2 3      …which runs Title's `on exit` then Play's `on enter`
+#     900      Play renders the same frame it is entered (Hud.Draw)
+#     201 900  frame 3: Play.World.Step, then Hud.Draw
+#     202 900  frame 4: Step reaches 2 -> quit(); Hud.Draw paints the last frame
+#
+#   bin/x game-build bin/ludicc examples/lang/scenes.ludic /tmp/scenes
+#   printf 'aaaa' | /tmp/scenes
+program SceneDemo {
+  var counter: int = 0
+
+  handler Boot phase Start { counter = 0;  print(1000) }
+
+  scene Title start {
+    on enter { print(1) }
+    on exit  { print(2) }
+    layer Main {
+      handler Tick phase Update {
+        counter = counter + 1
+        print(100 + counter)
+        if counter >= 2 { become Play }
+      }
+    }
+  }
+
+  scene Play {
+    on enter { print(3);  counter = 0 }
+    on exit  { print(4) }
+    layer World {
+      handler Step phase Update {
+        counter = counter + 1
+        print(200 + counter)
+        if counter >= 2 { quit() }
+      }
+    }
+    layer Hud {
+      handler Draw phase Render { print(900) }
+    }
+  }
+}