Add minimal transactional state manager package

[syifan]

Summary

• introduce a v5/sim/state package with a basic Manager supporting register/load/stage/commit flows
• provide unit tests covering staging, commit, discard, and example usage for documentation

Testing

• go test ./v5/sim/state

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https://chatgpt.com/codex/tasks/task_e_68e5c31354388325af6f7546f3f549d8

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prevent post-commit mutation of active state

After Stage returns a mutable pointer to the staged copy, Commit assigns that pointer directly to active (e.active = e.staged). Callers that keep the returned pointer can continue mutating it after the commit without calling Stage again, bypassing the transactional guard and allowing unsynchronized writes to the active state. Any goroutine that edits the previously staged object after Commit will mutate live state without triggering hasStaged, making rollbacks impossible and introducing data races with concurrent Load calls.

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