fix(engines): guard usePdfiumEngine success path against cancellation

The init effect only checked its `cancelled` flag on the error path, so
under React Strict Mode's dev mount/unmount/remount it created two engines
and called `setEngine` for both. The second engine identity remounts any
consumer keyed on the engine (e.g. `<EmbedPDF>`), abandoning an in-flight
document load, and the first engine leaks because the cleanup that ran saw
`engineRef.current === null`.

Guard the success path: if the effect was already torn down, destroy the
freshly created engine and return before touching state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: junwen-k

.changeset/pdfium-engine-strict-mode-guard.md

@@ -0,0 +1,5 @@
+---
+'@embedpdf/engines': patch
+---
+
+fix(engines): guard `usePdfiumEngine`'s success path against cancellation. The init effect only checked its `cancelled` flag on the error path, so under React Strict Mode's dev double-mount it created two engines and called `setEngine` for both — remounting engine-keyed consumers (e.g. `<EmbedPDF>`) mid-load and leaking the first engine. It now destroys the engine and bails if the effect was already torn down.

packages/engines/src/shared/hooks/use-pdfium-engine.ts

@@ -44,6 +44,20 @@ export function usePdfiumEngine(config?: UsePdfiumEngineProps) {
           encoderPoolSize,
           fontFallback,
         });
+
+        // The effect can be torn down before this resolves — notably under
+        // React Strict Mode, which mounts/unmounts/remounts in dev. Without
+        // this guard the success path runs after cleanup: it calls setEngine
+        // with a second engine identity (remounting engine-keyed consumers)
+        // and leaks this engine, because the cleanup that already ran saw
+        // engineRef.current === null.
+        if (cancelled) {
+          pdfEngine.closeAllDocuments?.().wait(() => {
+            pdfEngine.destroy?.();
+          }, ignore);
+          return;
+        }
+
         engineRef.current = pdfEngine;
         setEngine(pdfEngine);
         setLoading(false);