Add NetworkHealthWatchdog: agentic temporal graph health monitor

Monitors sequences of graph snapshots and detects structural anomalies:
- Density spikes/collapses vs rolling baseline
- Hub disappearance and emergence
- Component fragmentation
- Mass edge loss (>30% in one step)
- Mass node influx (>50% new nodes)
- Isolation waves

Outputs prioritized P0-P3 alerts with action playbook (owner, blast radius).
Health score 0-100, A-F grade, trajectory (improving/stable/declining).
Text, Markdown, and JSON renderers.

Usage:
  var watchdog = new NetworkHealthWatchdog();
  watchdog.addSnapshot("day-1", graph1);
  watchdog.addSnapshot("day-2", graph2);
  var report = watchdog.analyze();
  System.out.println(watchdog.toMarkdown(report));

Includes 14-case JUnit test suite.

Author: sauravbhattacharya001