Function group_name 

pub fn group_name(
    base: &str,
    namespace: &str,
    source_index: usize,
    repo_key: Option<&str>,
) -> String

The shared VolumeGroupSnapshot name for one expansion in one namespace.

Derived from the per-invocation base name, NOT from any one member: every member must compute the identical name without talking to its siblings, which is what makes their racing server-side-applies converge on one object instead of N.

Bounded to 63 chars for the same reason child names are — see fanout_child_name. repo_key adds the repository dimension for multi-repo fan-out — one VolumeGroupSnapshot per (repository, slot), because each repo’s members are an independent capture wave (independent-captures semantics: the N groups are N separate point-in-time CSI snapshots, at N× the CSI quota/load). Existing callers pass None and get the byte-identical legacy name.