…apierClusterSim

Adds the V2 surface so Rapier-backed nodes are usable for real games — without
this, integration is "uniform spheres with invisible collisions" (fine for tech
demo, useless for typical action gameplay).

Public additions:
- `RapierClusterSimulation` trait — sibling of `ClusterSimulation`, lives in
  `arcane-infra::rapier_cluster` so it can use Rapier types freely. Receives a
  `RapierClusterTickContext` instead of `ClusterTickContext`.
- `RapierClusterTickContext<'a>` — same fields as `ClusterTickContext` plus
  `contact_events: &[ContactEvent]` from the previous tick's physics step.
  One-tick delay by design: user logic runs first to set intent, physics produces
  output for next tick.
- `ContactEvent { entity_a, entity_b, started }` — collisions mapped from
  Rapier collider handles back to entity Uuids via a new reverse map.
- `RapierColliderShape::{Ball | Capsule | Cuboid}` — declared per entity via
  `RapierClusterSimulation::collider_for`. Default impl returns
  `Ball(config.default_body_radius)`. Resolved at first-sight spawn only;
  later returns are ignored (despawn-and-respawn to change shape).
- `RapierClusterSim::with_rapier_sim(rapier_sim, config)` constructor for the
  new trait. V1 `new` and `with_default_config` constructors preserved.

Internal refactor:
- `RapierClusterSim` now holds a private `Backend { None, Cluster, Rapier }` enum.
  `on_tick` dispatches per variant; the Rapier branch builds the extended ctx.
- `RapierState` gains `collider_to_entity: HashMap<ColliderHandle, Uuid>` for
  event mapping and `pending_contact_events: Vec<ContactEvent>` populated by a
  custom `EventHandler` impl (`CollisionRecorder`) installed during the step.
  Spawn loop and shape resolution moved out of `RapierState` into the wrapper
  so the active backend can drive `collider_for`.
- Every spawned collider sets `ActiveEvents::COLLISION_EVENTS`.

Tests:
- 6 new V2 tests: contact event surfaces for overlapping spheres; distant
  capsules produce no contacts; collider_for honored at first-sight spawn
  (verified via direct ColliderSet shape inspection); shape change after
  first-sight is ignored AND collider_for is called exactly once per entity;
  one-tick-delay semantics for contact events; no duplicate Started for a
  persistent overlap.
- All 18 V1 tests pass unchanged (same trait, same constructors, same wire).

Verification: 54 unit tests + 35 integration + 1 doctest pass under
`--features rapier-cluster`. Vanilla 65 tests pass; vanilla `cargo tree`
shows zero `rapier3d` references. Clippy silent both modes.

Closes #118.
Refs #8, #117.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

…ct tests

Synthesizes findings from the simplify skill (3 parallel review agents:
reuse / quality / efficiency) and a security-review pass, plus an
architectural pass on SemVer stability. Security review found zero
HIGH/MEDIUM vulnerabilities. Adds 14 tests so every module-doc claim is
backed by a test that would fail if the claim broke.

Code polish:
- New `to_rapier(Vec3) -> Vector` and `from_rapier(Vector) -> Vec3` helpers
  replacing five sites of triplet `.x as f32, .y as f32, .z as f32` casts.
- `RapierState::set_linvel` now takes `Vec3` instead of three `f64`s.
- Deleted unused `impl Default for RapierColliderShape` (closed a drift door
  vs `RapierConfig::default_body_radius`).
- `CollisionRecorder` propagates Mutex poison via `expect()` instead of
  silently dropping events on poison — surfaces panics that happen mid-step.
- Per-tick `pending_removals` lookup now uses a `HashSet` (was O(N×M) linear
  scan over a slice when entities × removals was non-trivial).
- Extracted `ClusterEnv::from_env()` helper into `cluster_runner`; both
  `arcane_cluster` and `arcane_rapier_cluster` binaries use it (was a
  verbatim duplicate of env-parsing across the two binaries).
- Stripped `v1`/`v2` release-stage labels from doc comments per project
  policy; trimmed several comments that restated the next line of code.

SemVer stability:
- `#[non_exhaustive]` on `RapierColliderShape`, `ContactEvent`,
  `RapierClusterTickContext`, `RapierConfig`. Future additions
  (e.g. `Cylinder` shape, `impulse_magnitude` on events, query handles in
  the context) won't be breaking changes.

New tests (every one corresponds to a documented contract that was
previously unverified):

T1 stopped_event_surfaces_when_bodies_separate
T2 despawn_during_contact_does_not_surface_stopped_event (pins the
   no-Stopped-on-despawn behavior; partners detect via the entity map)
T3 default_path_collider_is_a_ball_with_config_radius (V1 default shape
   directly inspected; previously only Cuboid was)
T4 capsule_collider_is_honored_at_first_sight (capsule shape inspected)
T5 multi_substep_in_one_cluster_tick (dt=0.1 → 6 substeps, position ~0.1)
T6 slow_dt_accumulates_until_substep_fires (dt=0.005, fires after ~3-4
   ticks, position converges to dt_total*v)
T7 contact_resolution_applies_impulse_to_partner (B gets pushed when A
   collides with it — Rapier responds, doesn't just detect)
T8 collider_for_invoked_freshly_on_respawn (despawn-respawn-same-uuid
   triggers a fresh shape decision)
T9 rapier_ctx_propagates_game_actions_tick_and_dt (V2 parallel of the V1
   context-propagation test)
T10 rapier_user_can_request_removal_via_pending_removals (V2 parallel of
    the V1 removal test)
T11 mixed_shape_ball_vs_cuboid_produces_contact (cross-shape collision
    now exercised; all prior contact tests paired same-shape)
T12 nondefault_gravity_honored_on_arbitrary_axis (gravity isn't hardcoded
    to -Y somewhere)
T13 contact_events_do_not_carry_across_handoff (cluster B's first tick
    sees ctx.contact_events == &[], not cluster A's events)
T14 capsule_axis_is_y (segment endpoints at (0, ±half_height, 0))

Verification: 68 lib tests (was 54) + 35 integration + 1 doctest pass under
`--features rapier-cluster`. Vanilla 65 unchanged. Clippy silent both modes.
Vanilla dep tree still has zero `rapier3d`.

Refs #117, #118, #8.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

… invariant; clarify body kinds in physics backends doc

New canonical doc `docs/architecture/entity-model.md` codifies the unified-
entity model decided in the 2026-05-03 architecture session:

- Arcane has one persistent-world concept: the Entity. Players, NPCs,
  projectiles, dropped items, structures, player-built walls — all are
  entities differentiated by per-entity hooks (body kind, collider, material,
  collision groups, sensor mode), not by separate types at the platform
  level. Matches modern engine practice (Unreal AActor, Unity GameObject,
  Bevy ECS, Godot Node).
- Two-axis classification (animate × moving/stationary) is described with
  industry-standard term cross-references.
- Physics body kinds (Dynamic / KinematicPositionBased / KinematicVelocityBased
  / Fixed) are documented with their per-tick cost and migration semantics.
- Affinity-bound vs spatial-bound distinction is called out as a clustering
  concern (not a physics concern); needs follow-up work in the clustering
  model so Fixed entities don't migrate by PGP affinity.
- Terrain is explicitly NOT entities — it's content loaded by the Arcane
  runtime based on entity positions. Cross-link to terrain epic #119.

Updates to `four-bucket-state-model.md`:
- Adds the "every entity has bucket-4 durable state" universal invariant up
  front. This is what makes recovery / migration work and what unifies
  ephemeral game objects with structural ones in a single concept.

Updates to `physics-backends-and-unreal.md` §6 (entity ↔ body mapping):
- Adds body-kind row (per-entity hook, default Dynamic).
- Adds explicit terrain-is-not-entities row with cross-link to #119.
- Notes Rapier's sleep mechanism + Fixed-body solver-skip preserve the
  "no entities → no simulation" invariant without needing a separate
  Structure concept.

Refs #117, #118, #119, #120, #121, #122, #8, #33.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

…in lines

CI's `cargo fmt --check` flagged six sites in rapier_cluster.rs (V2 tests with
long-form `entities.insert(id, mk_entry(...))` calls and one chained
with_collider closure) plus one site in cluster_runner.rs (long-line
Uuid::parse_str with map_err). Pure formatting; no functional changes.
All 38 rapier_cluster tests pass; clippy silent both modes.

Refs #117, #118, #123.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>