Mode-selective electron-phonon coupling drives charge density waves in the kagome metals YRu${}_3$Si${}_2$ and LaRu${}_3$Si${}_2$

YRu${}_3$Si${}_2$ and LaRu${}_3$Si${}_2$ host the highest charge density wave (CDW) transition temperatures ever reported in kagome metals, $T_{\mathrm{C}\mathrm{D}\mathrm{W}}\approx 800$ and 400~K, with propagation vectors $Q_{\mathrm{C}\mathrm{D}\mathrm{W}}\approx (1\mathrm{/}2,0,0)$ and $(1\mathrm{/}4,0,0)$, respectively. The microscopic mechanism behind these record values has remained unresolved. Here, we combine anharmonic phonon renormalization extracted from molecular dynamics driven by a machine-learned force field with mode-resolved electron-phonon coupling (EPC) analysis to identify this mechanism. The CDW is driven by mode-selective EPC: the Ru out-of-plane phonon modes couple the Ru-$d_{xz}\mathrm{/}{d}_{yz}$ and Si-$p_x\mathrm{/}{p}_y$ orbitals, and the phonon linewidth peaks sharply at $Q_{\mathrm{C}\mathrm{D}\mathrm{W}}$, while the featureless electronic susceptibility rules out Fermi-surface nesting. Phonon anharmonicity melts the CDW, and the anharmonic phonon spectra reproduce both $Q_{\mathrm{C}\mathrm{D}\mathrm{W}}$ and $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ in good agreement with experiments. Molecular dynamics simulations also visualize the CDW melting in real space. Chemical bonding analysis further shows that the smaller Y${}^{3+}$ radius strengthens the Ru-Ru bonds, enhancing lattice rigidity and accounting for the factor-of-two higher $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ of YRu${}_3$Si${}_2$. Our results establish a unified microscopic picture of CDW formation and melting in kagome metals YRu${}_3$Si${}_2$ and LaRu${}_3$Si${}_2$.