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>

tools/x/prelude.ludic

@@ -75,6 +75,21 @@ function getenv_or(name: pointer, dflt: pointer) -> pointer {
   return v
 }
 
+# flatten a relative example path into a filesystem-safe token: '/' -> '_', so a
+# categorised path like "games/snake" yields a single-segment temp name
+# ("games_snake") that never implies a missing /tmp subdirectory.
+function flat(p: pointer) -> pointer {
+  let n = len(p)
+  let b = bytes(n + 1)
+  for i in 0 .. n {
+    var c = p[i]
+    if (c == 47) { c = 95 }   # '/' (47) -> '_' (95)
+    b[i] = c
+  }
+  b[n] = 0
+  return b
+}
+
 # ---- stdout helpers ---------------------------------------------------------
 
 # write `s` with no trailing newline (print() always adds one)