Identifies one fetchable resource: the initialisation segment, a Low-Latency
HLS partial segment (“part”, RFC 8216bis §4.4.4.9), or a whole media
segment. Used to correlate an Action::FetchResource with the matching
crate::client::LlHlsClient::on_resource call.
Part/Segment are keyed by Media Sequence Number (RFC 8216 §4.3.3.2) +
(for parts) the 0-based Part Index within that segment — stable identity
independent of the URI, since a delta/blocking reload can re-describe the
same resource with the same URI more than once.
Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.