feat: modularize FSM core and add transactional Spring Boot example

[NGirchev]

Summary

• Split the reusable library into fsm-core; keep the transactional Spring Boot/Envers example isolated from publishing.
• Support per-transition immediate/deferred auto scheduling, serialization and bounded immediate auto-transition chains.
• Preserve @Version, synchronize detached versions after saves, and serialize workflow starts before invoking the external service.
• Fix review findings: copied transition/DTO scheduling metadata, cross-package non-public compatibility implementations, and the original Java/JVM builder overload.
• Keep the concurrent-start regression test compatible with Java 17 and enforce the Java 17 API at compilation, including when building on newer JDKs.
• Update vulnerable runtime/build/editor dependencies.

Verification

• Independent subagent review against master, with focused regression tests for the API fixes.
• ./gradlew check (core tests, quality/coverage gates and transactional example).
• Editor: npm test -- --run (59 tests), npm run build, npm audit --audit-level=moderate (0 vulnerabilities).
• GitHub CI passed on the current commit, including Java 17 compilation/tests, Detekt and coverage gates: https://github.com/NGirchev/fsm/actions/runs/35380521884.

Branch relationship

Related: #5 (feature/spring-boot-example), a separate PostgreSQL/versioned-flow example.
Neither branch contains the other; both started from the same master commit. This PR does not replace #5. Core layout/runtime and dependency changes overlap, so the second PR will need reconciliation after the first is merged.