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>

selfhost/parse.ludic

@@ -282,12 +282,14 @@ fn already_loaded(full: ptr) -> bool {
 fn parse_one_decl() -> void {
   let is_export = false
   let qspec: Node = ptr_null()
+  let onspawn_model: ptr = ptr_null()
   while is_op("@") {
     pi = pi + 1; let a = eat_id()             # collect a leading @annotation
     if streq(a, "export") { is_export = true }
-    if streq(a, "Queries") { qspec = parse_queries_anno() }   # @Queries(these: [...], on: ...)
+    else { if streq(a, "Queries") { qspec = parse_queries_anno() }   # @Queries(these: [...], on: ...)
+    else { if streq(a, "OnSpawn") { eat_op("("); onspawn_model = eat_id(); eat_op(")") }   # @OnSpawn(Model)
     else { if is_op("(") { let d = 0          # any other @anno(args) — parsed and skipped
-        while true { if is_op("(") { d = d + 1 };  if is_op(")") { d = d - 1 };  pi = pi + 1;  if d == 0 { break } } } }
+        while true { if is_op("(") { d = d + 1 };  if is_op(")") { d = d - 1 };  pi = pi + 1;  if d == 0 { break } } } } } }
     skipnl()
   }
   if is_id("import") { pi = pi + 1
@@ -303,6 +305,9 @@ fn parse_one_decl() -> void {
   if is_id("model") { push(prog, parse_archetype()); return }
   if is_id("handler") {
     let h = parse_system()
+    if not ptr_is_null(onspawn_model) {        # @OnSpawn(Model): a spawn hook, not a phased handler
+      register_onspawn(onspawn_model, h.a); return
+    }
     if not ptr_is_null(qspec) {               # @Queries wraps the body in its S_QUERY
       qspec.a = h.a
       let wrap = node(N_BLOCK); push(wrap.kids, qspec); h.a = wrap
@@ -346,6 +351,8 @@ fn maybe_splice_runtime() -> void {
 
 fn parse_program() -> void {
   prog = new []Node
+  g_computed = new []Node
+  g_onspawn = new []Node
   loaded_paths = new []ptr
   skipnl()
   g_game_name = "Ludic"