feat(render): add D2 diagram renderer with sequence, flow, and architecture styles

- Add D2Renderer with three styles: sequence, flow, arch
- Sequence diagrams use D2's sequence_diagram shape with phase grouping
- Flow diagrams show data flow with entity shapes (person, cylinder, hexagon)
- Architecture diagrams group entities by type (Users, Clients, Servers, etc.)
- Add FormatD2, FormatD2Flow, FormatD2Arch formats
- Update CLI help and README documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: grokify

README.md

@@ -32,7 +32,7 @@ PIDL models protocols as directed interaction graphs between entities, enabling
 ## Features
 
 - 📋 **JSON-based DSL** for describing protocol flows
-- 🎨 **Multiple output formats**: PlantUML, Mermaid, Graphviz DOT
+- 🎨 **Multiple output formats**: PlantUML, Mermaid, Graphviz DOT, D2
 - 📚 **Built-in examples**: OAuth 2.0, PKCE, OIDC, MCP, A2A
 - ⌨️ **CLI tool** for validation and diagram generation
 - 📦 **Go library** for programmatic use
@@ -70,6 +70,15 @@ pidl generate -f mermaid oauth2_pkce
 
 # Graphviz DOT data flow diagram
 pidl generate -f dot mcp_tool_invocation
+
+# D2 sequence diagram
+pidl generate -f d2 oauth2_pkce
+
+# D2 data flow diagram
+pidl generate -f d2-flow oauth2_pkce
+
+# D2 architecture diagram
+pidl generate -f d2-arch oauth2_pkce
 ```
 
 ### Create a new protocol file
@@ -171,10 +180,16 @@ Options:
 pidl generate [options] <file>
 
 Options:
-  -f string   Output format: plantuml, mermaid, dot (default "plantuml")
+  -f string   Output format: plantuml, mermaid, dot, d2, d2-flow, d2-arch (default "plantuml")
   -o string   Output file (default: stdout)
 ```
 
+D2 formats:
+
+- `d2` - Sequence diagram
+- `d2-flow` - Data flow diagram with entity shapes
+- `d2-arch` - Architecture diagram with entities grouped by type
+
 ### examples
 
 ```bash

cmd/pidl/main.go

@@ -113,7 +113,7 @@ Options:
 
 func cmdGenerate(args []string) {
 	fs := flag.NewFlagSet("generate", flag.ExitOnError)
-	formatStr := fs.String("f", "plantuml", "Output format: plantuml, mermaid, dot")
+	formatStr := fs.String("f", "plantuml", "Output format: plantuml, mermaid, dot, d2, d2-flow, d2-arch")
 	output := fs.String("o", "", "Output file (default: stdout)")
 	fs.Usage = func() {
 		fmt.Print(`Usage: pidl generate [options] <file>
@@ -128,6 +128,9 @@ Formats:
   plantuml, puml   PlantUML sequence diagram
   mermaid, mmd     Mermaid sequence diagram
   dot, graphviz    Graphviz DOT data flow diagram
+  d2               D2 sequence diagram
+  d2-flow          D2 data flow diagram
+  d2-arch          D2 architecture diagram
 `)
 	}