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/

doc.go

@@ -0,0 +1,26 @@
+// Package logz provides structured logging backed by [log/slog].
+//
+// Create a logger with [New]:
+//
+//	logger := logz.New(logz.Options{
+//	    Level: slog.LevelDebug,
+//	    JSON:  true,
+//	    StaticArgs: []any{"service", "api", "env", "production"},
+//	})
+//
+// Log at any level:
+//
+//	logger.Info("server started", "port", 8080)
+//	logger.Error("request failed", err, "path", "/users")
+//
+// Errors that implement ErrorCode() and ErrorContext() are automatically
+// enriched — the error code and context fields are added to the log record
+// without any extra method calls. This pairs naturally with xerrors.Err.
+//
+// Attach request context to a child logger:
+//
+//	ctx = logz.WithRequestID(ctx, requestID)
+//	ctx = logz.WithField(ctx, "user_id", userID)
+//	reqLogger := logger.WithContext(ctx)
+//	reqLogger.Info("handling request")
+package logz