Respect plot sizing metadata when present (#1119)

This change adds the ability to change the way plots are drawn using
four new values in the `positron` field of `execute_request`

- `fig-width` / `fig-height` from Quarto (takes precedence), from the
cell options
- `output_width_px` and `output_pixel_ratio` from the notebook and/or
source editor. These are device metrics that can change at any time.

Used together, these values cause most plots in notebooks and Quarto
documents to be drawn at the user's preferred size, and at the scaling
appropriate for their display.

Quarto:

<img width="764" height="466" alt="image"
src="https://github.com/user-attachments/assets/45053988-afbb-46ad-8b60-b3977efd72a6"
/>


Jupyter:

<img width="774" height="833" alt="image"
src="https://github.com/user-attachments/assets/c797ebd8-ad6d-465e-b632-9576cf4f3cd1"
/>

Author: jmcphers

crates/amalthea/src/wire/execute_request.rs

@@ -41,9 +41,23 @@ pub struct ExecuteRequest {
 }
 
 #[serde_with::skip_serializing_none]
-#[derive(Debug, Serialize, Deserialize, Clone)]
+#[derive(Debug, Serialize, Deserialize, Clone, Default)]
 pub struct ExecuteRequestPositron {
     pub code_location: Option<JupyterPositronLocation>,
+
+    /// Figure width in inches, set by Quarto
+    #[serde(rename = "fig-width")]
+    pub fig_width: Option<f64>,
+
+    /// Figure height in inches, set by Quarto
+    #[serde(rename = "fig-height")]
+    pub fig_height: Option<f64>,
+
+    /// Output area width in pixels
+    pub output_width_px: Option<f64>,
+
+    /// Device pixel ratio of the output area
+    pub output_pixel_ratio: Option<f64>,
 }
 
 #[derive(Debug, Serialize, Deserialize, Clone)]

crates/ark/src/console/console_repl.rs

@@ -1288,12 +1288,20 @@ impl Console {
                     reply_tx,
                 });
 
-                // Push execution context to graphics device for plot attribution.
+                // Push execution context to graphics device for plot attribution
+                // and optional sizing overrides from Quarto.
                 let code_location = exec_req.code_location().log_err().flatten();
+                let (render_settings, intrinsic_size) = exec_req
+                    .positron
+                    .as_ref()
+                    .map(graphics_device::compute_plot_overrides)
+                    .unwrap_or((None, None));
                 graphics_device::on_execute_request(
                     originator.header.msg_id.clone(),
                     exec_req.code.clone(),
                     code_location,
+                    render_settings,
+                    intrinsic_size,
                 );
 
                 input

crates/ark/src/modules/positron/graphics.R

@@ -421,11 +421,7 @@ finalize_device_arguments <- function(format, width, height, pixel_ratio) {
 #' "on screen" be as close to the size in which they are actually printed at,
 #' which has always been tricky.
 default_resolution_in_pixels_per_inch <- function() {
-    if (Sys.info()[["sysname"]] == "Darwin") {
-        96L
-    } else {
-        72L
-    }
+    .ps.Call("ps_graphics_default_dpi")
 }
 
 #' Determines the default device `type` for png, jpeg, and tiff