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>

docs/language/pool/pool-capacity.md

@@ -0,0 +1,14 @@
+---
+id: pool-capacity
+name: Pool.capacity
+category: pool
+kind: namespace-method
+tokens: Pool.capacity
+sig: Pool.capacity() -> int
+tip: The maximum number of entities.
+order: 4
+ns: Pool
+member: capacity
+---
+
+Returns the maximum number of live entities (the fixed entity-array size). Budget spawns against it — a horde/bullet-hell game keeps <a href="pool-live"><code>Pool.live</code></a> under <code>Pool.capacity()</code>.