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/world_mixed.ludic

@@ -1,17 +0,0 @@
-# world_mixed.ludic — EV2b reflection: get/set use real struct offsets, not a
-# field*4 assumption (replaces tests/mod_c/world_mixed.c). Slot's `qty` sits after
-# a `ptr` field, so its byte offset is 8, not 4; writing qty=99 by name and reading
-# it back proves the world table addresses fields by their true layout. Prints 99.
-program WorldMixed {
-  property Slot { ref: pointer = null, qty: int = 0 }
-  model Item { Slot }
-  @Public @OnSpawn(Item) handler Born { }          # makes it a modding program
-
-  entry {
-    let e   = world_spawn(world_model_id("Item"))
-    let S   = world_prop_id("Slot")
-    let qty = world_field_id(S, "qty")             # field index 1, after the ptr
-    world_set(e, S, qty, 99)
-    print(world_get(e, S, qty))                    # 99 iff the offset is 8, not 4
-  }
-}