fix(cdp,js): playwright actionability stubs and objectId resolve (#178)

Playwright fails locator actions on Obscura for two avoidable reasons
that are separate from the missing layout engine. Both have trivial
fixes that unblock the actionability polling loop.

- Element.checkVisibility(opts) was undefined. Playwright >= 1.25 calls
  it before every input event and retries until its action timeout if
  the method isn't a function. Add a stub that returns true. Without a
  layout pass we can't compute it for real, but a permissive stub still
  beats the undefined-function path.

- ARIA reflection properties were missing on Element (role, ariaLabel,
  ariaSelected, ariaExpanded, ariaHidden, ariaChecked, ariaDisabled,
  ariaRoleDescription). Playwright's getByRole and getByLabel locators
  read them. Reflect through to the underlying aria-* attributes for
  both read and write.

- DOM.getBoxModel and DOM.getContentQuads ignored the objectId param.
  Playwright passes only objectId (from a prior DOM.resolveNode), and
  the handlers silently defaulted to node 0, so dispatchMouseEvent
  always clicked the wrong element. Extract a resolve_node_id helper
  that honors nodeId, backendNodeId, and objectId in that order.

- Page.captureScreenshot and Page.captureSnapshot now return a clear
  "not supported, use a real Chromium for the screenshot leg" error
  instead of the generic "Unknown Page method" fallback. Matches the
  pattern already in place for Page.printToPDF.

Partial fix for #45. The architectural blocker (no layout engine, so
multi-element click coordinates are wrong) is unchanged.

crates/obscura-cdp/src/domains/dom.rs

@@ -1,8 +1,36 @@
+use obscura_browser::Page;
 use obscura_dom::{DomTree, NodeData, NodeId};
 use serde_json::{json, Value};
 
 use crate::dispatch::CdpContext;
 
+/// Resolve a DOM `nodeId` from CDP params. Honors `nodeId`, `backendNodeId`,
+/// and `objectId` in that order. Playwright commonly passes only `objectId`
+/// (returned by a prior `DOM.resolveNode`); without this fallback those
+/// requests silently default to node 0 and click the wrong element.
+fn resolve_node_id(page: &mut Page, params: &Value) -> Result<u64, String> {
+    if let Some(nid) = params.get("nodeId").and_then(|v| v.as_u64()) {
+        return Ok(nid);
+    }
+    if let Some(nid) = params.get("backendNodeId").and_then(|v| v.as_u64()) {
+        return Ok(nid);
+    }
+    if let Some(oid) = params.get("objectId").and_then(|v| v.as_str()) {
+        let code = format!(
+            "(function() {{ var o = globalThis.__obscura_objects && globalThis.__obscura_objects['{}']; \
+             return (o && typeof o._nid === 'number') ? o._nid : -1; }})()",
+            oid.replace('\'', "\\'")
+        );
+        let result = page.evaluate(&code);
+        let nid = result.as_f64().map(|n| n as i64).unwrap_or(-1);
+        if nid < 0 {
+            return Err(format!("objectId {oid} could not be resolved to a node"));
+        }
+        return Ok(nid as u64);
+    }
+    Err("nodeId, backendNodeId, or objectId required".to_string())
+}
+
 pub async fn handle(
     method: &str,
     params: &Value,
@@ -138,10 +166,8 @@ pub async fn handle(
         "setAttributeValue" => Ok(json!({})),
         "removeNode" => Ok(json!({})),
         "getBoxModel" => {
-            let node_id = params.get("nodeId").and_then(|v| v.as_u64())
-                .or_else(|| params.get("backendNodeId").and_then(|v| v.as_u64()))
-                .unwrap_or(0);
             let page = ctx.get_session_page_mut(session_id).ok_or("No page")?;
+            let node_id = resolve_node_id(page, params)?;
             let code = format!(
                 "(function() {{\
                     var el = globalThis._wrap && globalThis._wrap({0});\
@@ -175,10 +201,8 @@ pub async fn handle(
             }))
         }
         "getContentQuads" => {
-            let node_id = params.get("nodeId").and_then(|v| v.as_u64())
-                .or_else(|| params.get("backendNodeId").and_then(|v| v.as_u64()))
-                .unwrap_or(0);
             let page = ctx.get_session_page_mut(session_id).ok_or("No page")?;
+            let node_id = resolve_node_id(page, params)?;
             let code = format!(
                 "(function() {{\
                     var el = globalThis._wrap && globalThis._wrap({0});\

crates/obscura-cdp/src/domains/page.rs

@@ -313,6 +313,17 @@ pub async fn handle(
                     .to_string(),
             )
         }
+        "captureScreenshot" | "captureSnapshot" => {
+            // Same story as printToPDF: rasterising a page needs a layout and
+            // paint pipeline that Obscura intentionally does not have. Reply
+            // with a clear error so clients can fail fast instead of waiting
+            // on the generic "Unknown Page method" reply.
+            Err(format!(
+                "Page.{method} is not supported by Obscura: no layout or paint engine. \
+                 For visual snapshots, drive a real headless Chromium for the \
+                 screenshot leg of your pipeline and use Obscura for the scraping leg."
+            ))
+        }
         _ => Err(format!("Unknown Page method: {}", method)),
     }
 }
@@ -398,4 +409,32 @@ mod tests {
             "error must point to a workaround: {err}"
         );
     }
+
+    /// Regression for #45: same idea as printToPDF for captureScreenshot.
+    /// Playwright's `page.screenshot()` calls Page.captureScreenshot via CDP;
+    /// without an explicit arm, clients see "Unknown Page method" and have
+    /// no idea why their screenshot request failed.
+    #[tokio::test]
+    async fn capture_screenshot_returns_descriptive_unsupported_error() {
+        let mut ctx = CdpContext::new();
+        let err = handle("captureScreenshot", &json!({}), &mut ctx, &None)
+            .await
+            .expect_err("captureScreenshot must error until a real paint exists");
+        assert!(
+            !err.contains("Unknown Page method"),
+            "captureScreenshot must NOT fall through to the catch-all: {err}"
+        );
+        assert!(
+            err.contains("not supported by Obscura"),
+            "error must clearly state screenshot is unsupported: {err}"
+        );
+        // Same for the MHTML snapshot sibling method.
+        let err2 = handle("captureSnapshot", &json!({}), &mut ctx, &None)
+            .await
+            .expect_err("captureSnapshot must error until a real renderer exists");
+        assert!(
+            !err2.contains("Unknown Page method"),
+            "captureSnapshot must NOT fall through: {err2}"
+        );
+    }
 }