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>

tools/x/test.ludic

@@ -208,6 +208,8 @@ function cmd_test() -> int {
   feat_case("library/render", "", "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18", "render.ludic (Screen pixel/oval/camera/clip/blend_mode/measure_text + Camera set/follow/shake, verified by pixel readback)")
   feat_case("library/lighting", "", "1 2 3 4 5 6 7 8 9 10 11 12 13 14", "lighting.ludic (Light ambient/point radial falloff + occluder hard shadows — 2D light accumulation, verified by pixel readback)")
   feat_case("library/light_tiers", "", "1 1 1 1 1 1 1 1 1", "light_tiers.ludic (Light spot/falloff/soft/gel/normal/time_of_day — render-quality tiers 3-4; issue #49)")
+  feat_case("library/physics", "", "184 1 1 284 1 1 50 51 104 1 134 1 120 0 1 1 1 0 1 1 1", "physics.ludic (Body + Collider + esys_move: gravity/land, wall stop, sub-pixel, entity platform, one-way, triggers — entity broadphase; issue #65)")
+  feat_case("library/physics_tiles", "", "112 1 80 1 48 1 60", "physics_tiles.ludic (esys_move tile-grid broadphase: tile floor/wall + one-way tile land-from-above / rise-through; issue #65)")
   spec_case("library/testing", "== 6 passed, 0 failed ==")
   spec_case("library/coverage", "== 3 passed, 0 failed ==")
   feat_case("library/errors", "", "5 10 0 7 1", "errors.ludic (assert guards an invariant, holds -> runs to the end; issue #8 success path)")