feat(ecs): #80 entity-pool stats (Pool.live/free/reserved/capacity)

Ludic's ECS is already pool-based — the allocator recycles freed entity slots
through a freelist (L_alloc pops @L_freen before growing @L_entc), and component
storage is fixed per-entity arrays, so spawn/despawn churn (bullet-hell/horde)
does no per-spawn heap allocation and cannot fragment. Expose that with a Pool.*
namespace so a game can watch reuse: Pool.live (alive now), Pool.free (recycled
slots waiting), Pool.reserved (high-water — stays flat across a steady
spawn/despawn loop, proving reuse not reallocation), Pool.capacity (the fixed
cap). Zero-cost inline reads of the existing counters.

Example pool.ludic proves the key property: after despawn+respawn,
Pool.reserved() stays 3 (freed slot reused) — prints 0 0 3 3 0 2 1 3 0 3 1.
4 docs pages. Full suite 118/0, goldens byte-identical, fixpoint holds.

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

examples/library/pool.ludic

@@ -0,0 +1,39 @@
+# pool.ludic — entity pooling stats (#80). Ludic's ECS already recycles freed
+# entity slots through a freelist (a despawned slot is reused by the next spawn
+# before any new slot is taken), so bullet-hell / horde spawn+despawn does no
+# per-spawn allocation and cannot fragment. Pool.* exposes the counters so a game
+# can watch the reuse. The key proof: after despawn + respawn, Pool.reserved()
+# does NOT grow — the freed slot was reused, not reallocated.
+#
+# Deterministic; a full run prints:  0 0 3 3 0 2 1 3 0 3 1
+program PoolDemo {
+  property Mob { hp: int = 0 }
+  model M { Mob }
+
+  function bi(b: bool) -> int { if b { return 1 };  return 0 }
+
+  entry {
+    print(Pool.reserved())               # 0 — nothing allocated yet
+    print(Pool.live())                   # 0
+
+    spawn M { Mob { hp: 1 } }
+    spawn M { Mob { hp: 2 } }
+    spawn M { Mob { hp: 3 } }
+    print(Pool.reserved())               # 3
+    print(Pool.live())                   # 3
+    print(Pool.free())                   # 0 — no freed slots yet
+
+    let pm = World.prop_id("Mob")
+    let e = World.query_next(pm, 0)
+    despawn e
+    print(Pool.live())                   # 2
+    print(Pool.free())                   # 1 — one slot recycled onto the freelist
+
+    spawn M { Mob { hp: 4 } }             # reuses the freed slot
+    print(Pool.reserved())               # 3 — NO new slot allocated (pooled, not reallocated)
+    print(Pool.free())                   # 0 — the freelist slot was drawn back down
+    print(Pool.live())                   # 3
+
+    print(bi(Pool.capacity() >= 3))      # 1 — there is a fixed entity cap to budget against
+  }
+}