No std mutex

[Person-93]

This PR is to prepare for no_std support.

I've added non_poison::Mutex and Once to the platform mod.

There was once place that relied on poisoning for correctness: err_state.rs, but that didn't really need a mutex because it was guarding a Copy type, I've replaced it with a Cell.

[codspeed-hq]

Merging this PR will improve performance by 30.65%

⚡ 1 improved benchmark
✅ 124 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	drop_many_objects_without_gil	48.2 µs	36.9 µs	+30.65%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing Person-93:no_std_mutex (e2e9ad0) with main (6d0c1ee)}