docs: clarify lifetime and ownership rules for C hook callback parameters

[kinyoklion]

Strengthen the documentation in hook.h to make it explicit that all SDK-provided pointers (series_context, data, detail, etc.) must not be retained, cached, or stored beyond the scope of the callback invocation, and that any data needed outside the callback must be copied.

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Note

Low Risk
Documentation-only updates that clarify ownership/lifetime rules for SDK-provided callback parameters; no runtime behavior changes.

Overview
Adds explicit lifetime/ownership warnings to the C hook API docs (hook.h), stating that all SDK-provided pointers (series_context, data, detail, etc.) are temporary and must not be retained beyond the callback.

Tightens the per-callback LIFETIME/RETURNS sections to consistently note that any needed data must be copied within the callback and that returned EvaluationSeriesData ownership transfers to the SDK.

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[beekld]

I'm curious to hear what prompted the extra warning, but always a good reminder.

LGTM