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>

.gitignore

@@ -1,11 +1,9 @@
-# Generated build tree: LLVM IR, objects, compiled apps and the docs site all
-# land under build/ (see `bin/x build` / `bin/x clean`). Root-anchored so a
-# source dir named "build" elsewhere is never accidentally ignored.
+# Generated build tree: LLVM IR, objects, compiled apps, the headless render
+# (build/out.ppm) and the docs site all land under build/ (see `bin/x build` /
+# `bin/x clean`). Root-anchored so a source dir named "build" elsewhere is never
+# accidentally ignored. Nothing is written to the repo root any more.
 /build/
 
-# Headless render dropped in the working directory by a --headless run.
-/out.ppm
-
 # Archives are never tracked: the Kenney art is committed *extracted* under
 # assets/kenney/**/ (PNG/tmx/License), while the original download .zips and any
 # packaged plugin .zip are local-only build inputs/outputs.