gpu: Capturer::on_device and Renderer::report

A capturer on a device already open (the app's), and the renderer's
JSON shared by every reply.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

Cargo.lock

@@ -1379,6 +1379,7 @@ dependencies = [
  "fieldcraft-render",
  "log",
  "pollster 0.4.0",
+ "serde_json",
  "thiserror 2.0.21",
  "wgpu",
 ]

crates/gpu/Cargo.toml

@@ -13,6 +13,7 @@ fieldcraft-render = { workspace = true }
 wgpu = { workspace = true }
 log = { workspace = true }
 thiserror = { workspace = true }
+serde_json = { workspace = true }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 pollster = { workspace = true }

crates/gpu/src/capture.rs

@@ -61,6 +61,12 @@ impl Capturer {
         .map_err(|e| GpuError::Device(e.to_string()))?;
         // Errors outside a scope are logged, replacing wgpu's default handler, which panics.
         device.on_uncaptured_error(Arc::new(|e: wgpu::Error| log::error!("graphics device error: {e}")));
+        Capturer::on_device(adapter, device, queue, tier)
+    }
+
+    /// A capturer on a device already open on `adapter` (the desktop app's), which keeps its
+    /// error handler: pictures share the window's device instead of opening another.
+    pub fn on_device(adapter: &wgpu::Adapter, device: wgpu::Device, queue: wgpu::Queue, tier: Tier) -> Result<Capturer> {
         let samples = if adapter.get_texture_format_features(FORMAT).flags.sample_count_supported(SAMPLES)
             && adapter.get_texture_format_features(DEPTH).flags.sample_count_supported(SAMPLES)
         {

crates/gpu/src/lib.rs

@@ -69,4 +69,13 @@ impl Renderer {
             Renderer::Cpu { .. } => "cpu",
         }
     }
+
+    /// What drew a picture, for a reply: `{kind, adapter, msaa}` on a GPU, `{kind, why}` on
+    /// the CPU (`why` null when the CPU was asked for).
+    pub fn report(&self) -> serde_json::Value {
+        match self {
+            Renderer::Gpu { adapter, samples, .. } => serde_json::json!({"kind": self.kind(), "adapter": adapter, "msaa": samples}),
+            Renderer::Cpu { why } => serde_json::json!({"kind": self.kind(), "why": why}),
+        }
+    }
 }

crates/mcp/src/backend.rs

@@ -35,10 +35,7 @@ fn draw(f: &fieldcraft_engine::view::Framed, w: u32, h: u32) -> Result<(Vec<u8>,
 /// the adapter, or why the CPU drew it), `results` (step, scale, contour), `undrawn_bodies` and
 /// `sampled_symbols`. The MCP `screenshot` reply and `fieldcraft-cli snapshot` print it.
 pub fn picture_report(r: &Renderer, shown: &Shown) -> Value {
-    let renderer = match r {
+    let renderer = r.report();
-        Renderer::Gpu { adapter, samples, .. } => json!({"kind": r.kind(), "adapter": adapter, "msaa": samples}),
-        Renderer::Cpu { why } => json!({"kind": r.kind(), "why": why}),
-    };
     let contour = shown
         .contour
         .as_ref()