Add pkg.go.dev examples for all public APIs

[Copilot]

The package lacks Go examples for pkg.go.dev, making API discovery and usage patterns unclear compared to sibling projects (lifecycle, loam, procio, trellis).

Changes

Added example_test.go with 11 testable examples covering:

Generic APIs (domain-agnostic)

• Example() - Basic TypedWatcher usage and state observation
• ExampleTreeDiagram() - Hierarchical structure visualization
• ExampleComponentDiagram() - Component relationship diagrams
• ExampleStateMachineDiagram() - State machine visualization
• ExampleAggregateWatchers() - Multi-component state aggregation
• ExampleNewWatcherAdapter() - TypedWatcher → StateSnapshot conversion
• ExampleAggregateEvents() - Multi-source event aggregation
• ExampleWithStyles() - Custom Mermaid styling

Legacy APIs (worker/signal domain)

• ExampleWorkerTreeDiagram() - Worker hierarchy visualization
• ExampleSignalStateMachine() - Signal handler state machine
• ExampleSystemDiagram() - Combined signal/worker topology

Example Usage

// Define your domain state
type ServiceState struct {
    Name   string
    Status string
}

// Create a watcher
service := &MyService{state: ServiceState{Name: "API", Status: "Running"}}

// Watch for changes
ctx := context.Background()
changes := service.Watch(ctx)
for change := range changes {
    fmt.Printf("State: %s -> %s\n", change.OldState.Status, change.NewState.Status)
}

All examples are deterministic (pass -race detector) and use minimal, self-contained code. Helper types included for copy-paste usage.

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