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/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}"
+        );
+    }
 }