docs: restructure README newbie-first + fact-fix against source

Reorder so setup comes first: TL;DR ("What you get") and Security & trust
lead, then prerequisites/install/launch, then the sandbox test flights,
then the feature catalog, tool reference, architecture, troubleshooting.

Corrections verified against JDWPTools.java / pom.xml / .mcp.json:
- tool count 46 → 47 (jdwp_reconnect, jdwp_dump_locks added since last edit)
- Connection table 3 → 4, Inspection table 8 → 9
- document forceLoad (deferred-by-default vs force-bind + <clinit>),
  get_threads verbose, overview showEmpty, block-mode { } expressions,
  resume_until_event TIMEOUT/VM_DEATH/NO_EVENT envelopes + soft-wait
- new sections: jdwp_dump_locks lock graph, jdwp_reconnect
- architecture/structure: add JdiHealthMonitor, DeadlockAnalyzer,
  marks/, discovery/, transport/ packages; "47 @mcptool"
- prerequisites: JDK 17+ to run, JDK 21+ to build (Error Prone 2.48)
- Spring Boot 4.0 → 4.0.5

Author: novoj