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/networking/net_echo.ludic

@@ -0,0 +1,24 @@
+# net_echo.ludic — N0: the transport seam (NETWORKING-DESIGN §5, §13 N0).
+#
+# The transport is two calls — net_send puts a datagram on the wire, net_poll
+# takes the next one off. A production build binds them to a real socket with
+# `extern fn net_send/net_poll` (UDP native, WebRTC/WebSocket wasm); absent that,
+# the compiler supplies a built-in in-process loopback, so a program is networked
+# end to end with NO foreign host — pure Ludic. This sends four bytes and polls
+# them back through the loopback: prints 4, then 10 20 30 42.
+program NetEcho {
+  entry {
+    let out = bytes(4)
+    out[0] = 10
+    out[1] = 20
+    out[2] = 30
+    out[3] = 42
+    net_send(0, out, 4)                  # onto the wire (the built-in loopback)
+
+    let inb = bytes(64)
+    let n = net_poll(inb, 64)            # take the next datagram back off
+    print(n)                             # 4
+    var i = 0
+    while i < n { print(inb[i]);  i = i + 1 }   # 10 20 30 42
+  }
+}