Store, freeze, and propagate DSC on transactions #1161

Change transaction handling so DSC is stored on the transaction instead of being constructed only at finish time. TransactionContext::continue_from_headers should parse both sentry-trace and baggage, incoming DSC should be treated as frozen, TransactionInner should store the chosen DSC, transaction envelopes should use the stored DSC, and TraceHeadersIter should yield baggage in addition to sentry-trace when a DSC is available.

Preserve current sentry-trace behavior for callers without baggage, and keep this issue limited to DSC lifecycle and propagation rather than new ClientOptions fields or the strict-continuation decision matrix. Related: #741, #1016.