Scriptable properties (ECS-safe subset): @Computed and @OnSpawn

Adds two annotations that give properties/models a scriptable feel WITHOUT
reattaching behavior to data — both reduce to code the data-oriented model
already emits:

- @Computed field on a property: a derived value that is NOT stored; x.field
  expands inline to its expression (bare names read as fields of x) at each use.
  Zero storage, zero runtime dispatch. Reuses qualify_fields (now non-destructive,
  base-node based); a g_computed registry keeps derived fields out of the layout.
- @OnSpawn(Model) on a handler: a constructor that runs at each spawn of Model
  with the model's properties bound by name. Spawn statically knows the model, so
  no runtime dispatch; emit_spawn binds the properties and inlines the hook body.

examples/annotations.ludic now exercises @Handles/@Queries/@Computed/@OnSpawn
(output 3 25 0 0); test.sh 15/15, fixpoint holds, goldens byte-identical.

Deferred (need more machinery, by design): @OnDespawn (despawn doesn't statically
know the entity's model) and @OnChange (needs change-tracking).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test.sh

@@ -50,10 +50,11 @@ qsmoke() {  # name
   else bad "$n: $(tail -1 /tmp/qs.out)"; fi
 }
 qsmoke qdecl
-# @Queries desugars to a query loop; @Handles parses. Check the desugared behavior.
+# The annotation DSL: @Queries desugars to a query, @Computed inlines a derived
+# field, @OnSpawn runs a constructor, @Handles parses. Check the composite output.
 if ./selfhost/game-build.sh build/ludicc examples/annotations.ludic "/tmp/ludic_ann" >/tmp/ann.out 2>&1 \
-   && [ "$(/tmp/ludic_ann | tr '\n' ' ')" = "3 1 0 0 " ]; then
-  ok "annotations.ludic (@Queries desugars to a query, @Handles parses)"
+   && [ "$(/tmp/ludic_ann </dev/null | tr '\n' ' ')" = "3 25 0 0 " ]; then
+  ok "annotations.ludic (@Queries / @Computed / @OnSpawn / @Handles)"
 else bad "annotations: $(tail -1 /tmp/ann.out)"; fi
 # --- Toolchain-agent CLI smoke tests append below this line ---
 echo "== self-hosted front-end binaries (ludicc / ludic) =="