[HaukeRa]

Two defects in the viewport buffer path killed consuming apps in the field (Kotlin/Native turns an uncaught worker exception into process death):

1. ConsumerBufferPool.getBufferViewport disposed the previous viewport buffer inline whenever a larger viewport was requested — with no coordination with an in-flight render still holding it (PdfView renders in a debounced produceState coroutine on Dispatchers.IO, so zoom/rotation restarts it mid-flight) or with the displayed image, which references the buffer's pixels zero-copy via extractSubset. Calling withAddress on the disposed buffer dies natively (SIGABRT in SkBitmap::peekPixels on JVM, SIGSEGV on iOS); the silent variant has the native renderer writing through a dangling pointer. This is likely the root cause of An error is reported when zooming a PDF on the Android side #41 ("recycled bitmap" while zooming on Android — the same race in the Android buffer implementation).

   Replaced buffers are now retired instead of disposed and cleaned up on a later request once their consumer freed them. Handed-out viewport buffers are marked unfree; PdfViewport releases the replaced image (buffer-identity guarded, since same-size requests reuse the instance).

2. ConsumerBufferUtil.allocate ignored allocPixels' Boolean result, so a failed allocation (memory pressure, oversized viewport at deep zoom) handed out a pixel-less bitmap whose peekPixels() returns null — surfacing much later as a misleading "Could not read bitmap buffer address" inside a render. Observed in production via crash reporting on iOS. Allocation failures now fail fast with a descriptive message, and PdfState contains buffer failures into its existing error state (CancellationException rethrown) instead of letting them escape the render coroutine.

New nonAndroidConsumerTest suite covers fail-fast allocation, held- buffer survival on viewport growth, the retire/free/dispose lifecycle, and unfree marking; the existing consumerBufferTest expectation is updated to the new lifecycle. Verified on jvmTest and iosSimulatorArm64Test.