Release 1.2.0: add logged-test module and JSON event emission

[Fayupable]

• Add logged-test module: in-memory, thread-safe test doubles for all three output ports (InMemoryInvocationEventEmitter, InMemoryMetricsRecorder with a new RecordedMetric record, InMemoryClientInfoPort), so a consuming project can assert directly on @Logged behavior in tests instead of parsing log output or hand-rolling its own stubs. Depends only on logged-core, works with or without logged-spring on the classpath.
• Add JsonInvocationEventEmitter (logged-spring): writes each MethodInvocationEvent as a single line of structured JSON through SLF4J, for applications shipping logs to Loki/Elasticsearch/Datadog and similar backends that parse each line as JSON. Uses a small hand-written JSON writer with proper string escaping instead of adding a JSON library dependency.
• Update README with "Testing with logged-test" and "JSON event emission" sections, and the Modules table.

Bumps version to 1.2.0.