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/library/arena.ludic

@@ -0,0 +1,34 @@
+# ============================================================================
+# arena.ludic — a game that links the Ludic shared library next to it.
+#
+#   ludicc examples/library/combat.ludic --shared -o build/libcombat.dylib
+#   ludicc examples/library/arena.ludic  -o build/arena -Lbuild -lcombat
+#
+# `extern fn` binds a name to a symbol resolved at link time. The library the
+# symbols come from happens to be written in Ludic, but nothing here depends on
+# that: the same declaration binds a C, Rust or Zig library just as well. This
+# is how Ludic reaches the outside world without a C shim.
+# ============================================================================
+program Arena {
+
+  extern function damage(attack: int, armour: int, roll: int) -> int = "damage"
+  extern function hits_to_kill(hp: int, attack: int, armour: int) -> int = "hits_to_kill"
+  extern function xp_for(level: int, kills: int) -> int = "xp_for"
+
+  property Fighter { hp: int = 30, attack: int = 9, armour: int = 2 }
+
+  handler Boot phase Start {
+    spawn Hero { Fighter { hp: 40, attack: 12, armour: 3 } }
+    spawn Orc  { Fighter { hp: 24, attack: 7,  armour: 1 } }
+  }
+
+  handler Fight phase Update {
+    for (f) in query [Fighter] {
+      print(damage(f.attack, f.armour, 0))
+      print(damage(f.attack, f.armour, 20))
+      print(hits_to_kill(f.hp, f.attack, f.armour))
+    }
+    print(xp_for(3, 4))
+    quit()
+  }
+}

examples/library/combat.ludic

@@ -0,0 +1,46 @@
+# ============================================================================
+# combat.ludic — a Ludic *library*, not a game.
+#
+# `module` instead of `game` means: no entry point, no frame loop. ludicc
+# compiles this to a real shared object —
+#
+#     ludicc examples/library/combat.ludic --shared -o build/libcombat.dylib
+#
+# — whose `export fn`s are ordinary C-ABI symbols. Anything that can call a
+# .dylib/.so/.dll can call these: another Ludic program via `extern fn`, a game
+# engine, a test harness, a scripting host. Non-exported functions stay private
+# to the library.
+# ============================================================================
+program Combat {
+
+  const CRIT_MULT: int = 2
+
+  # damage after armour, with a crit multiplier applied on an exact roll
+  @export function damage(attack: int, armour: int, roll: int) -> int {
+    var raw = attack - armour
+    if raw < 1 { raw = 1 }
+    if roll == 20 { raw = raw * CRIT_MULT }
+    return raw
+  }
+
+  # how many hits to drop a target — the loop is here so the caller cannot
+  # accidentally disagree with `damage` about rounding
+  @export function hits_to_kill(hp: int, attack: int, armour: int) -> int {
+    var left = hp
+    var n = 0
+    while left > 0 {
+      left = left - damage(attack, armour, 0)
+      n = n + 1
+    }
+    return n
+  }
+
+  @export function xp_for(level: int, kills: int) -> int {
+    return curve(level) * kills
+  }
+
+  # private: not exported, so it is not a symbol in the library
+  function curve(level: int) -> int {
+    return 10 + level * level * 3
+  }
+}