Decouple internal buffered state from final StateVectorTrialResult and ensure that qsimcirq can return the state vector directly

[jkalsi1]

Is your feature request related to a use case or problem? Please explain

See reference #8041. This is a suggested refactor from @sergeisakov

How urgent is this for you? Is it blocking important work?

P3 – I'm not really blocked by it; it's an idea I'm proposing based on principle

Provide additional information or context (e.g., screenshots)

See also: quantumlib/qsim#893