fix(flow): improve tail capture and final chunk ASR recovery

- Add FlowUtteranceEndCoordinator with 350ms silence drain and 150ms post-roll
- Extend FinalChunkRecovery for short/empty final chunks in chunked pipeline
- Snapshot partial at mic stop and guard final transcript in FlowSessionManager
- Unify tail drain presets (iosFlow/macMLX) and expand diagnostics

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

OSGKeyboardShared/Utilities/FinalChunkRecovery.swift

@@ -0,0 +1,47 @@
+// FinalChunkRecovery.swift
+// OSGKeyboard · Shared
+//
+// Recovery plans for pipelined utterance ASR when the final chunk is short,
+// empty, or straddles a chunk boundary.
+
+import Foundation
+
+public enum FinalChunkRecovery {
+
+    /// Samples and stitch index when the final chunk should be merged with
+    /// prior overlap *before* the first ASR pass.
+    public static func preMergePlan(
+        chunk: UtteranceAudioChunk,
+        processedChunks: Int,
+        previousChunkSamples: [Float],
+        config: FlowUtteranceChunkConfig
+    ) -> (samples: [Float], stitchIndex: Int)? {
+        guard chunk.isLast, !chunk.samples.isEmpty, !previousChunkSamples.isEmpty else {
+            return nil
+        }
+        guard chunk.samples.count < config.minFinalChunkSamples else { return nil }
+
+        let merged = Array(previousChunkSamples.suffix(config.overlapSamples)) + chunk.samples
+        return (merged, max(0, chunk.index - 1))
+    }
+
+    /// Retry plan when the final chunk had audio but ASR returned empty text.
+    public static func emptyResultRetryPlan(
+        chunk: UtteranceAudioChunk,
+        previousChunkSamples: [Float],
+        config: FlowUtteranceChunkConfig,
+        asrText: String
+    ) -> (samples: [Float], stitchIndex: Int)? {
+        guard chunk.isLast, !chunk.samples.isEmpty else { return nil }
+        guard asrText.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty else {
+            return nil
+        }
+
+        if previousChunkSamples.isEmpty {
+            return (chunk.samples, chunk.index)
+        }
+
+        let merged = Array(previousChunkSamples.suffix(config.overlapSamples)) + chunk.samples
+        return (merged, max(0, chunk.index - 1))
+    }
+}