feat(flow): add whole-utterance batch ASR fallback (P1)

- Accumulate utterance PCM in FlowContinuousCapture for batch retry
- Run full-utterance transcribeChunk when stitched final lags partial
- Refactor Mac MLX tail drain to shared FlowUtteranceEndCoordinator
- Add FlowUtterancePCMStore, UtteranceBatchFallbackPolicy, and tests

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

OSGKeyboardShared/Utilities/FlowUtterancePCMStore.swift

@@ -0,0 +1,46 @@
+// FlowUtterancePCMStore.swift
+// OSGKeyboard · Shared
+//
+// Thread-safe rolling buffer of 16 kHz mono utterance PCM for whole-utterance
+// batch ASR fallback when pipelined chunking drops weak tail segments.
+
+import Foundation
+
+public final class FlowUtterancePCMStore: @unchecked Sendable {
+    private let lock = OSAllocatedUnfairLock()
+    private var samples: [Float] = []
+    private let maxSampleCount: Int
+
+    public init(maxSampleCount: Int) {
+        self.maxSampleCount = max(1, maxSampleCount)
+    }
+
+    public func reset() {
+        lock.withLock {
+            samples.removeAll(keepingCapacity: false)
+        }
+    }
+
+    public func append(_ chunk: [Float]) {
+        guard !chunk.isEmpty else { return }
+        lock.withLock {
+            samples.append(contentsOf: chunk)
+            if samples.count > maxSampleCount {
+                samples.removeFirst(samples.count - maxSampleCount)
+            }
+        }
+    }
+
+    public var sampleCount: Int {
+        lock.withLock { samples.count }
+    }
+
+    /// Returns accumulated samples and clears the store.
+    public func consume() -> [Float] {
+        lock.withLock {
+            let out = samples
+            samples.removeAll(keepingCapacity: false)
+            return out
+        }
+    }
+}