fisherMetric50 (diagonal) = fisherMetric (bilinear) on full space.
On tangent space, they differ by 1/p_0 cross-term.
Bridge: full-space metric is determined by tangent-space restriction.
chentsov_theorem gives uniqueness on tangent space.
Lifting to full space is standard linear algebra.
chentsov_50 sorry updated with correct SplitEmbedding type
and h_pos hypothesis. Bridge requires chentsov_theorem (3 internal sorries).
