Function compare 

pub fn compare(
    baseline: &CtprofSnapshot,
    candidate: &CtprofSnapshot,
    opts: &CompareOptions,
) -> CtprofDiff

Compare two snapshots and produce a CtprofDiff. Sequences the comparison phases in data-flow order: build the per-side thread groups, emit matched + one-sided rows, fudge one-sided cgroups together (producing the fudged_key_pairs consumed by the uptime and enrichment phases), sort the one-sided lists, fill uptime%, order the rows, and attach enrichment. The fudged-pair (bkey, ckey) threading is the only cross-phase data dependency: built by apply_cgroup_fudge, read by fill_uptime_pct and attach_enrichment.