Skip error recovery on intentional cancellation (#477)

When SlidingWindowAsrManager is cancelled, CancellationError propagates
through processWindow() and the audio buffer loop. Previously this
triggered attemptErrorRecovery() which resets the decoder and, as a
last resort, re-downloads models — neither of which is appropriate for
an intentional shutdown.

Guard both catch sites with `error is CancellationError || Task.isCancelled`
to return immediately instead.

Fixes #477

https://claude.ai/code/session_01696MyMtoiM6T8ruCdCCHab

Sources/FluidAudio/ASR/Parakeet/SlidingWindow/SlidingWindowAsrManager.swift

@@ -159,6 +159,9 @@ public actor SlidingWindowAsrManager {
                     // Append to raw sample buffer and attempt windowed processing
                     await self.appendSamplesAndProcess(samples)
                 } catch {
+                    if error is CancellationError || Task.isCancelled {
+                        return
+                    }
                     let streamingError = SlidingWindowAsrError.audioBufferProcessingFailed(error)
                     logger.error(
                         "Audio buffer processing error: \(streamingError.localizedDescription)")
@@ -470,6 +473,9 @@ public actor SlidingWindowAsrManager {
             updateContinuation?.yield(update)
 
         } catch {
+            if error is CancellationError || Task.isCancelled {
+                return
+            }
             let streamingError = SlidingWindowAsrError.modelProcessingFailed(error)
             logger.error("Model processing error: \(streamingError.localizedDescription)")