test(04-02): add i3bar formatting with TDD

- I3BarBlock struct for i3bar protocol JSON
- formatI3BarBlocks: compact format D-06/D-07/D-08
- 10 tests covering mixed states, colors, labels, markers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

i3bar.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// I3BarBlock represents a single block in the i3bar protocol.
+type I3BarBlock struct {
+	FullText  string `json:"full_text"`
+	ShortText string `json:"short_text,omitempty"`
+	Color     string `json:"color"`
+	Name      string `json:"name"`
+	Markup    string `json:"markup,omitempty"`
+}
+
+// formatI3BarBlocks builds the vmux status block for i3bar from session data.
+func formatI3BarBlocks(sessions []SessionInfo) []I3BarBlock {
+	if len(sessions) == 0 {
+		return []I3BarBlock{{FullText: "vmux: no sessions", Color: "#00ff00", Name: "vmux"}}
+	}
+
+	hasWaiting := false
+	parts := make([]string, 0, len(sessions))
+
+	for _, s := range sessions {
+		name := shortName(s)
+		switch s.State {
+		case "Needs Input":
+			parts = append(parts, name+"[!]")
+			hasWaiting = true
+		case "Working":
+			parts = append(parts, name+"[W]")
+		case "Idle":
+			parts = append(parts, name+"[I]")
+		}
+	}
+
+	var text string
+	if !hasWaiting {
+		text = fmt.Sprintf("vmux: all working (%d)", len(sessions))
+	} else {
+		text = "vmux: " + strings.Join(parts, " ")
+	}
+
+	color := "#00ff00"
+	if hasWaiting {
+		color = "#ff0000"
+	}
+
+	return []I3BarBlock{{FullText: text, Color: color, Name: "vmux"}}
+}