feat(logz): initial stable release v0.9.0

Structured logger backed by log/slog with request-context enrichment, extra-field context helpers, and duck-typed automatic error enrichment.

What's included:
- `Logger` interface with Debug / Info / Warn / Error / With / WithContext; `New(Options)` constructor writing to os.Stdout
- `WithRequestID` / `GetRequestID` and `WithField` / `WithFields` context helpers — package owns both context keys
- Automatic error_code and context-field enrichment in Logger.Error via duck-typed errorWithCode / errorWithContext interfaces (no xerrors import)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

context.go

@@ -0,0 +1,43 @@
+package logz
+
+import "context"
+
+// ctxRequestIDKey is the context key for the request correlation ID.
+type ctxRequestIDKey struct{}
+
+// ctxExtraFieldsKey is the context key for arbitrary logging fields.
+type ctxExtraFieldsKey struct{}
+
+// WithRequestID adds a request correlation ID to the context.
+func WithRequestID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, ctxRequestIDKey{}, id)
+}
+
+// GetRequestID retrieves the correlation ID from the context.
+// Returns "" if not present or if ctx is nil.
+func GetRequestID(ctx context.Context) string {
+	if ctx == nil {
+		return ""
+	}
+	id, _ := ctx.Value(ctxRequestIDKey{}).(string)
+	return id
+}
+
+// WithField adds a single key-value pair to the context for logging.
+func WithField(ctx context.Context, key string, value any) context.Context {
+	return WithFields(ctx, map[string]any{key: value})
+}
+
+// WithFields adds multiple key-value pairs to the context for logging.
+// Merges with any existing fields — does not overwrite the whole map.
+func WithFields(ctx context.Context, fields map[string]any) context.Context {
+	existing, _ := ctx.Value(ctxExtraFieldsKey{}).(map[string]any)
+	newMap := make(map[string]any, len(existing)+len(fields))
+	for k, v := range existing {
+		newMap[k] = v
+	}
+	for k, v := range fields {
+		newMap[k] = v
+	}
+	return context.WithValue(ctx, ctxExtraFieldsKey{}, newMap)
+}