Simplify namespaces proposal

[imurashka]

One root namespace makes the package cleaner to import and in line with what most Unity‑side libs already do.

• Fewer using lines – drop the extra directives and cut the noise.
• Matches common practice – packages like UniRx and Zenject expose a single top‑level namespace, so devs expect it.
• Quicker API discovery – IDE autocomplete shows the whole surface at once.
• Lower risk of name clashes – a unified prefix keeps types well‑scoped.