feat(log): add colored terminal output without external packages (#16)

Adds ANSI color-coded log levels for terminal output:
- TRACE: Gray
- DEBUG: Cyan
- INFO: Green
- NOTICE: Blue
- WARN: Yellow
- ERROR/PANIC/FATAL: Red (bold for PANIC/FATAL)

Colors are auto-detected based on terminal capability and can be
controlled via SetColorEnabled()/ColorEnabled().

Fixes #9

log/color.go

@@ -0,0 +1,99 @@
+package log
+
+import (
+	"os"
+	"sync"
+)
+
+// ANSI color codes for terminal output
+const (
+	colorReset  = "\033[0m"
+	colorRed    = "\033[31m"
+	colorGreen  = "\033[32m"
+	colorYellow = "\033[33m"
+	colorBlue   = "\033[34m"
+	colorPurple = "\033[35m"
+	colorCyan   = "\033[36m"
+	colorWhite  = "\033[37m"
+	colorGray   = "\033[90m"
+
+	// Bold variants
+	colorBoldRed    = "\033[1;31m"
+	colorBoldYellow = "\033[1;33m"
+	colorBoldWhite  = "\033[1;37m"
+)
+
+var (
+	colorEnabled     = true
+	colorEnabledOnce sync.Once
+	colorMux         sync.RWMutex
+)
+
+// SetColorEnabled enables or disables colored output for stderr logging.
+// By default, color is enabled when stderr is a terminal.
+func SetColorEnabled(enabled bool) {
+	colorMux.Lock()
+	colorEnabled = enabled
+	colorMux.Unlock()
+}
+
+// ColorEnabled returns whether colored output is currently enabled.
+func ColorEnabled() bool {
+	colorMux.RLock()
+	defer colorMux.RUnlock()
+	return colorEnabled
+}
+
+// isTerminal checks if the given file descriptor is a terminal.
+// This is a simple heuristic that works on Unix-like systems.
+func isTerminal(f *os.File) bool {
+	stat, err := f.Stat()
+	if err != nil {
+		return false
+	}
+	return (stat.Mode() & os.ModeCharDevice) != 0
+}
+
+// initColorEnabled sets the default color state based on whether stderr is a terminal.
+func initColorEnabled() {
+	colorEnabledOnce.Do(func() {
+		colorEnabled = isTerminal(os.Stderr)
+	})
+}
+
+// levelColor returns the ANSI color code for a given log level.
+func levelColor(level Level) string {
+	switch level {
+	case LTrace:
+		return colorGray
+	case LDebug:
+		return colorCyan
+	case LInfo:
+		return colorGreen
+	case LNotice:
+		return colorBlue
+	case LWarn:
+		return colorYellow
+	case LError:
+		return colorRed
+	case LPanic:
+		return colorBoldRed
+	case LFatal:
+		return colorBoldRed
+	default:
+		return colorReset
+	}
+}
+
+// colorize wraps text with ANSI color codes if color is enabled.
+func colorize(text string, color string) string {
+	if !ColorEnabled() {
+		return text
+	}
+	return color + text + colorReset
+}
+
+// colorizeLevelText returns the level string with appropriate color.
+func colorizeLevelText(level string, lvl Level) string {
+	return colorize(level, levelColor(lvl))
+}