Refactor atomics to enforce API-based access with relaxed load/store

[devreal]

Summary

• Wrap C11 _Atomic types in structs and provide OPAL_ATOMIC_VAR_INIT
• Add opal_atomic_load/store macros using relaxed memory order with explicit variants
• Update code to use new atomic API instead of direct loads/stores

Testing

• make check (fails: No rule to make target 'check')

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https://chatgpt.com/codex/tasks/task_b_68a39d62d08c8326a82cb77b2a41878c

[github-actions]

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9a8fd8d: Wrap atomic types with structs and add relaxed loa...

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