This commit updates the ECDH and ECDHXOnly functions to utilize optimized windowed multiplication instead of constant-time multiplication. This change is justified as the secret key is already known, allowing for variable-time operations. Additionally, new .idea configuration files are added, including .gitignore, misc.xml, modules.xml, p256k1.mleku.dev.iml, and vcs.xml, to enhance project management and version control settings.
