landgraph 0.0.1

• Initial release. Shared landscape-genetic primitives extracted from terradish so that the symmetric (terradish) and asymmetric (dragonflow) network methods stand on a common, dependency-light base.
• Graph: deme_graph() builds a lightweight deme/landscape graph (vertex coordinates + undirected edge list) from coordinates, with Delaunay, k-nearest- neighbour, or lattice (rook/queen) adjacency. The result is class c("landgraph", "terradish_graph").
• Genetic covariance / distance: cov_from_biallelic(), cov_from_genetic_data(), fst_from_biallelic(), dist_from_cov(), dist_from_biallelic().
• Directional edge covariates: edge_gradient() (gradient of a scalar potential) and edge_flow() (projection of a vector flow field, carrying a curl component).
• cov_from_biallelic() applies the per-locus pooled frequency correctly per column (fixes a column-major recycling error that affected multi-deme covariances).