[huashengshao]

Native consumers need the symmetric-adjoint color pair that appears when a gluon splits into a quark–antiquark pair. Add GluonSplittingColorPair, built from the existing ordered SplitGluon and EmitGluon operations, together with ColorConnection::apply_connected so existing exact tensor sums retain their complex coefficients, Laurent powers in Nc, and inverse-root-two normalizations.

The convention is all outgoing, with the physical splitting tensor S_ij,B = t^B_ij and T_R = 1/2. The helper exposes the raw daughter–spectator overlaps and computes D_parent . T_spectator = 2*(raw_quark - raw_antiquark), where (D^c)_AB = d^{AcB} has no factor of i; no additional T_R, real projection, or color averaging is applied. It validates an adjoint parent, a distinct colored spectator, fresh daughter/emission labels, and matching input domains. Mixed spin densities and kinematic splitting kernels remain the caller's responsibility.

Validation: cargo test -p rusticol-core --features symbolica-runtime color:: --lib --offline -j2 passed all 20 selected tests. New checks compare against an independent raw-UFO d-tensor network (Nc^3/4 - 5*Nc/4 + 1/Nc, giving 10/3 at SU(3)); recover the parent dipole from the normalized daughter sum for quark, antiquark and gluon spectators; preserve directed complex phases and original normalizations; and reject invalid domains or unrepresentable normalization parity. Changed Rust files are formatted and the diff passes whitespace checks.

This change is based on the native UFO-color support in PR #11 (commit 66ecab3b).