Publish the library, KSP processor and Gradle plugin to Maven Central #375

ROADMAP 15.1: nothing is published to a remote repository. repo1.maven.org has no org.thisisthepy/python-multiplatform*; the only distribution is a zipped Maven repo on the GitHub Release (desktop-only preview). The repo already has CENTRAL_TOKEN_USERNAME/PASSWORD and MAVEN_GPG_PRIVATE_KEY/PASSPHRASE secrets (2026-10-24), and the org.thisisthepy namespace is verified (the compose fork publishes there via extended-publish.yml).

Done when: a tag-triggered workflow signs and publishes python-multiplatform (all KMP targets: desktop, android, iosArm64/iosSimulatorArm64/iosX64, androidNative, macosArm64, linuxX64, wasmJs), python-multiplatform-ksp and python-multiplatform-gradle-plugin (including the plugin marker) to Maven Central under one version, and a consumer project outside the repo resolves them from mavenCentral() with no mavenLocal().