Store, freeze, and propagate DSC on transactions

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](https://github.com/getsentry/sentry-rust/issues/741), [#1016](https://github.com/getsentry/sentry-rust/issues/1016).