Add primary node customization hooks to ComponentDiagram

[Copilot]

Problem

ComponentDiagram primary nodes are hardcoded to use reflection on Enabled/Stopping/Stopped/Reason fields. This blocks lifecycle project migration which needs custom metadata display (signal counts) and CSS control (failure differentiation).

Changes

New customization hooks mirroring secondary node pattern:

type PrimaryNodeStyleFunc func(state any) (cssClass string)
type PrimaryNodeLabelFunc func(state any) string

type DiagramConfig struct {
    // ... existing fields ...
    PrimaryNodeStyler  PrimaryNodeStyleFunc
    PrimaryNodeLabeler PrimaryNodeLabelFunc
}

Default implementations encapsulate existing reflection logic:

• defaultPrimaryNodeStyler - extracts Enabled/Stopping/Stopped → CSS class
• defaultPrimaryNodeLabeler - extracts Reason → HTML label

Refactored renderGenericFragment to accept callbacks, falling back to defaults when nil (full backward compatibility).

Usage

// Custom state with domain-specific fields
type SignalState struct {
    Signal   string
    Received int
    Failed   bool
}

config := DefaultDiagramConfig()
config.PrimaryNodeStyler = func(state any) string {
    st := state.(SignalState)
    if st.Failed { return "failed" }
    if st.Signal == "SIGTERM" { return "stopping" }
    return "running"
}
config.PrimaryNodeLabeler = func(state any) string {
    st := state.(SignalState)
    return fmt.Sprintf("Signal: %s<br/>Received: %d", st.Signal, st.Received)
}

diagram := ComponentDiagram(signalState, processState, config)

Enables arbitrary state structures while maintaining zero-config defaults for legacy code.

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