SerialkompatPlugin

Registers the serialkompat compatibility-gate tasks (design §9):

• serialkompatExtract runs the extractor on the project's own runtime classpath (via JavaExec, so it sees the real compiled @Serializable types) and writes the current schema snapshot.

• serialkompatCheck recomputes the baseline from the target git ref (git-ref-live, GitRefBaseline) and compares it to the current schema, failing on an unacknowledged breaking change. Wired into check.