feat(engine): 2D collision / physics-lite — Body + Collider + esys_move (#65)

Phase 0 of the gameplay-controller work (#57): turn the static Collision.*
overlap tests into a real physics-lite moving-body-with-collision layer that
the platformer / shooter / NPC-AI / RPG families build on.

New engine-owned system esys_move (runtime/native/systems_move.ludic), spliced
and Update-phase-registered when a game declares `Body` (parse.ludic), standing
entirely on the reflection ABI like the SpriteAnim / Motion / Light2D systems:

- Body { vx, vy, gravity, max_fall, rx, ry, policy, on_ground, hit_wall,
  hit_ceiling } — Q16.16 velocity + engine-owned sub-pixel accumulators.
- Collider { w, h, offx, offy, is_trigger, one_way, layer, mask, hit, entered,
  exited } — AABB shape off Position, layer/mask filtering, one-way + triggers.
- Solids { tile, wall, oneway } — optional config entity enabling the tile-grid
  broadphase over the Map.* tilemap.

esys_move integrates velocity + gravity, then resolves per-axis swept AABB
against both solid Collider entities and the tile grid (no tunneling), handles
one-way platforms (block only a downward landing), reports trigger/sensor
overlaps without resolving, and sets on_ground / hit_wall / hit_ceiling for a
controller to poll. No float, no hidden singletons, no runtime dispatch —
integer + deterministic, so replay / lockstep / world_save hold. Contact is
surfaced as polled flags (the SpriteAnim.event_fired shape), so a game raises
its own CollisionResolved / TriggerEntered events with no engine coupling.

Two self-asserting examples (entity + tile broadphase) wired into `x test`;
docs added to the collision section. A build with no Body compiles byte-for-byte
the same (bootstrap fixpoint + all golden renders unchanged).

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

selfhost/frontend/parse.ludic

@@ -776,6 +776,13 @@ function maybe_splice_runtime() -> void {
     if (find_comp("SpriteAnim") != null) or (find_comp("Motion") != null) {
       do_import("runtime/native/systems.ludic")
     }
+    # Body -> the movement + collision system (#65). It reads the tilemap
+    # (rt_tile / rt_mapw / rt_maph) for the tile-grid broadphase, so it links
+    # against core.ludic; do_import dedupes when a game already pulled core in.
+    if find_comp("Body") != null {
+      do_import("runtime/native/core.ludic")
+      do_import("runtime/native/systems_move.ludic")
+    }
     # Light2D / Occluder -> the lighting render system, which links against the
     # 2D light pass (light.ludic). do_import dedupes, so this is a no-op when the
     # game also uses Light.* directly (g_uses_light already pulled it in).
@@ -824,6 +831,7 @@ function parse_program() -> void {
   g_esys_comp = new []pointer;  g_esys_fn = new []pointer;  g_esys_phase = new []pointer
   push(g_esys_comp, "SpriteAnim");  push(g_esys_fn, "esys_spriteanim");  push(g_esys_phase, "Update")
   push(g_esys_comp, "Motion");      push(g_esys_fn, "esys_motion");      push(g_esys_phase, "Update")
+  push(g_esys_comp, "Body");        push(g_esys_fn, "esys_move");        push(g_esys_phase, "Update")
   push(g_esys_comp, "Light2D");     push(g_esys_fn, "esys_light2d");     push(g_esys_phase, "Render")
   g_namespaces = new []pointer
   loaded_paths = new []pointer