Fetch Z3 distribution during build

[k0lja]

Summary

• download the platform-specific Z3 distribution during copyNativeLibs instead of keeping Z3 release zips in the repository
• depend on the Maven-published JavaSMT Z3 solver artifact instead of the local Z3 jar file tree
• keep SV-COMP packaging wired to the prepared runtime libraries without extracting vendored Z3 archives

Verification

• ./gradlew copyNativeLibs
• ./gradlew build -x test
• ./gradlew :symbolic-executor:test
• direct targets/examples/spring-1 local-solver run without a z3 executable on PATH or in the worktree:
  • command -v z3 produced no output
  • find /root/.openclaw/agents/hiwi/workspace/SWAT-issue7 -path '*/.git' -prune -o -type f -name z3 -print produced no output
  • direct java ... -Dconfig.path=swat-local.cfg ... run on port 18080 produced a Solution (LocalSolver) for /api/users/safe-search?username=user1&unfuzzable=10
  • pgrep -a z3 produced no output during the run

Fixes #7