feat(stdlib): add Query.* — ECS spatial queries over the reflection ABI (#42)

Completes the half of #24 that was explicitly deferred as blocked: entity-space
queries to sit alongside the grid-space Grid.*/pathfinding that shipped in
07e5a20. Query.* answers questions about the live entities that carry a
property, built directly on the EV2 reflection ABI (world_query_next/world_get):

  - Query.count(prop) -> int                 how many live entities carry prop
  - Query.first(prop) -> int                 the lowest-id bearer, or -1
  - Query.nearest(prop, pos, xf, yf, x, y)   the bearer closest to (x,y), or -1
  - Query.within(prop, pos, x, y, r, xf, yf) -> []int   every bearer within r

prop is a property id (World.prop_id); the spatial forms read a position from a
coordinate property `pos` at two int field ids (World.field_id), so `prop` can be
a discriminating tag distinct from the position component ("nearest Enemy"), or
the same id to query the coordinate component itself. Distances are exact squared
integers (no sqrt), ties break to the lower entity id, and `within` returns
entities in ascending id order — so every answer is deterministic and replay-safe.

The engine (runtime/native/query.ludic, ~55 lines of Ludic, C-free) is a linear
scan over the entity table — ample for the entity counts Ludic targets, the same
reasoning as the grid pathfinder's open set; a bucketed/quadtree index is a
future optimisation, not a correctness need. It is spliced on demand when the
parser sees Query.* (g_uses_query), which also force-emits the reflection ABI so
a Query program needs no @events of its own (previously the ABI required them).

examples/library/query.ludic asserts 18 cases over five entities at known
positions (count/first with a component filter, nearest with a separate tag vs
position property, within radii incl. r=0 and the empty-property case), wired
into x test (now 63 passed). Docs: a Query section + 4 per-symbol pages,
inventory/coverage green. Seed reseeded; the C-free bootstrap fixpoint holds.

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

selfhost/frontend/parse.ludic

@@ -162,6 +162,7 @@ function p_postfix() -> Node {
   while true {
     if is_op(".") { pi = pi + 1; let m = node(E_MEMBER); m.a = e; m.s = eat_id(); e = m
       if e.a.kind == E_ID and e.a.s == "Regex" { g_uses_regex = true }   # splice the regex runtime on demand
+      if e.a.kind == E_ID and e.a.s == "Query" { g_uses_query = true }   # splice the ECS spatial-query runtime on demand
     }
     else { if is_op("[") { pi = pi + 1; let lo = expr()
         if is_op("..") { pi = pi + 1; let sl = node(E_SLICE); sl.a = e; sl.b = lo; sl.c = expr(); eat_op("]"); e = sl }   # s[a..b] substring
@@ -374,6 +375,7 @@ function path_join(dir: pointer, rel: pointer) -> pointer {
 var loaded_paths: []pointer
 var cur_dir: pointer
 var g_uses_regex: bool = false    # a program mentioned Regex.* -> splice the regex runtime
+var g_uses_query: bool = false    # a program mentioned Query.* -> splice the query runtime + reflection ABI
 
 function already_loaded(full: pointer) -> bool {
   var i = 0
@@ -525,6 +527,14 @@ function maybe_splice_runtime() -> void {
     do_import("runtime/native/regex_vm.ludic")
     cur_dir = saved
   }
+  # any program that uses Query.* gets the ECS spatial-query helpers spliced in;
+  # they read entity state through the reflection ABI (emit_decl force-emits it
+  # for a Query program even when it declares no events).
+  if g_uses_query {
+    cur_dir = ""
+    do_import("runtime/native/query.ludic")
+    cur_dir = saved
+  }
 }
 
 function parse_program() -> void {
@@ -543,6 +553,7 @@ function parse_program() -> void {
   g_onlisten = new []Node
   g_toggled_layers = new []pointer
   g_uses_regex = false
+  g_uses_query = false
   loaded_paths = new []pointer
   skipnl()
   g_game_name = "Ludic"