feat(logz)!: promote to v1.0.0 — configurable io.Writer output destination

Add Writer io.Writer field to Options; when nil, defaults to os.Stdout (no
breaking change). Enables log capture in tests via the public API and supports
writing to files, multi-writers, or any io.Writer implementation in production.
All remaining roadmap items validated in production (xerrors enrichment
concurrency, WithFields merge semantics, Logger interface finality). API
committed as stable.

logz.go

@@ -3,6 +3,7 @@ package logz
 import (
 	"context"
 	"errors"
+	"io"
 	"log/slog"
 	"os"
 )
@@ -20,7 +21,7 @@ type errorWithContext interface {
 }
 
 // Options configures a Logger instance.
-// The zero value is valid: INFO level, text output, no static args.
+// The zero value is valid: INFO level, text output, os.Stdout, no static args.
 type Options struct {
 	// Level is the minimum log level. Default: slog.LevelInfo (zero value).
 	Level slog.Level
@@ -28,6 +29,9 @@ type Options struct {
 	JSON bool
 	// StaticArgs are key-value pairs attached to every log record.
 	StaticArgs []any
+	// Writer is the output destination. Defaults to os.Stdout when nil.
+	// Accepts any io.Writer: *os.File, bytes.Buffer, io.MultiWriter, etc.
+	Writer io.Writer
 }
 
 // Logger is the interface for structured logging.
@@ -56,16 +60,17 @@ type slogLogger struct {
 
 // New returns a new Logger configured by opts.
 func New(opts Options) Logger {
-	handlerOpts := &slog.HandlerOptions{
-		Level:     opts.Level,
-		AddSource: false,
+	w := opts.Writer
+	if w == nil {
+		w = os.Stdout
 	}
 
+	handlerOpts := &slog.HandlerOptions{Level: opts.Level}
 	var handler slog.Handler
 	if opts.JSON {
-		handler = slog.NewJSONHandler(os.Stdout, handlerOpts)
+		handler = slog.NewJSONHandler(w, handlerOpts)
 	} else {
-		handler = slog.NewTextHandler(os.Stdout, handlerOpts)
+		handler = slog.NewTextHandler(w, handlerOpts)
 	}
 
 	base := slog.New(handler)