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

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

crates/rapier_testbed3d-f64/Cargo.toml

@@ -51,7 +51,6 @@ rand_pcg = "0.3"
 web-time = { version = "1.1" }
 bitflags = "2"
 num_cpus = { version = "1", optional = true }
-crossbeam = "0.8"
 bincode = "1"
 md5 = "0.7"
 Inflector = "0.11"