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/main.ludic

@@ -27,12 +27,13 @@ program X {
     print("  x app <file.ludic> [--headless] [--save-temps]")
     print("                                compile a Ludic program to a native app in build/")
     print("  x tools [--install] [--test]  build the editor toolchain (ludic-fmt, ludic-lsp)")
-    print("  x clean                       remove build/, out.ppm and stray artifacts (keeps bin/)")
+    print("  x clean                       remove build/ (incl. build/out.ppm) and stray artifacts (keeps bin/)")
     print("")
     print("test:")
     print("  x test                        the full regression suite")
     print("  x selfhost-test               the self-hosting suite (correctness + bootstrap fixpoints)")
     print("  x test-tools                  the editor-toolchain suite")
+    print("  x golden                      regenerate selfhost/golden/renders.sha256 (review with git diff)")
     print("")
     print("self-host internals:")
     print("  x selfhost-build [ludicc] [out]   assemble + compile the self-host compiler")
@@ -61,6 +62,7 @@ program X {
     if (cmd == "test") { exit(cmd_test()) }
     if (cmd == "selfhost-test") { exit(cmd_selfhost_test()) }
     if (cmd == "test-tools") { exit(cmd_test_tools()) }
+    if (cmd == "golden") { exit(cmd_golden()) }
     if (cmd == "bootstrap") { exit(cmd_bootstrap()) }
     if (cmd == "bootstrap-cfree") { exit(cmd_bootstrap_cfree()) }
     if (cmd == "reseed") { exit(cmd_reseed()) }