How confident the resolver is in an Edge — the precision marker that lets
consumers (e.g. a quality analyzer) gate on resolution quality.
Variants are ordered from least to most precise:
Heuristic < NameOnly < Scoped < Exact.
More-precise compares greater, so consumers can write threshold filters such
as edge.confidence >= Confidence::Scoped to drop NameOnly edges, or
edge.confidence >= Confidence::NameOnly to drop the lowest Heuristic tier.
Lowest tier: a synthesized or normalized-name guess (e.g. case-folded
name match). Present so consumers can opt into maximum recall, never
dressed as a precise fact — filter it out for strict precision.