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/rendering/draw_prims.ludic

@@ -0,0 +1,13 @@
+# draw_prims.ludic — exercises the extended Screen.* drawing primitives
+# (line/circle/fill_circle/triangle/fill_triangle) for the golden render suite.
+program DrawPrims {
+  handler DrawWorld phase Render {
+    Screen.clear(0x0d1020)
+    Screen.line(x1: 4, y1: 4, x2: 120, y2: 60, color: 0xFF4040)
+    Screen.circle(x: 200, y: 60, radius: 30, color: 0x40FF40)
+    Screen.fill_circle(x: 80, y: 150, radius: 24, color: 0x4060FF)
+    Screen.triangle(x1: 160, y1: 120, x2: 230, y2: 130, x3: 190, y3: 200, color: 0xFFD040)
+    Screen.fill_triangle(x1: 250, y1: 150, x2: 300, y2: 160, x3: 270, y3: 210, color: 0xFF40FF)
+    Screen.show()
+  }
+}