Replace crossbeam channel with std::sync::mpsc (#861)

* Replace crossbeam channel with std::sync::mpsc

- Replace all uses of crossbeam::channel with std::sync::mpsc
- Remove crossbeam dependency from all Cargo.toml files
- Update documentation to remove crossbeam references
- Use std::sync::mpsc::channel() instead of crossbeam::channel::unbounded()

Fixes #828

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* Update mod.rs

---------

Co-authored-by: Claude <noreply@anthropic.com>

src_testbed/physics/mod.rs

@@ -1,4 +1,3 @@
-use crossbeam::channel::Receiver;
 use rapier::dynamics::{
     CCDSolver, ImpulseJointSet, IntegrationParameters, IslandManager, MultibodyJointSet,
     RigidBodySet,
@@ -8,6 +7,7 @@ use rapier::geometry::{
 };
 use rapier::math::{Real, Vector};
 use rapier::pipeline::{PhysicsHooks, PhysicsPipeline};
+use std::sync::mpsc::Receiver;
 
 pub struct PhysicsSnapshot {
     timestep_id: usize,