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>

OSGKeyboardTests/FlowUtteranceEndCoordinatorTests.swift

@@ -0,0 +1,35 @@
+// FlowUtteranceEndCoordinatorTests.swift
+// OSGKeyboardTests
+
+import XCTest
+@testable import OSGKeyboardShared
+
+final class FlowUtteranceEndCoordinatorTests: XCTestCase {
+
+    func testAwaitTailCaptureRunsPostRollAfterSilenceDrain() async {
+        let policy = FlowCaptureTailDrainPolicy(
+            silenceRMSThreshold: 0.02,
+            silenceDurationSeconds: 0.05,
+            maxDrainSeconds: 1.0,
+            postRollSeconds: 0.08
+        )
+        let tracker = FlowCaptureDrainTracker()
+        let start = Date().timeIntervalSince1970
+        tracker.beginDrain(now: start)
+
+        let timing = await FlowUtteranceEndCoordinator.awaitTailCapture(
+            tracker: tracker,
+            policy: policy,
+            pollIntervalNs: 5_000_000
+        )
+
+        XCTAssertTrue(timing.endedBySilence)
+        XCTAssertGreaterThanOrEqual(timing.postRollDurationSeconds, 0.07)
+    }
+
+    func testIOSFlowPresetUsesLongerSilenceAndPostRoll() {
+        XCTAssertEqual(FlowCaptureTailDrainPolicy.iosFlow.silenceDurationSeconds, 0.35)
+        XCTAssertEqual(FlowCaptureTailDrainPolicy.iosFlow.postRollSeconds, 0.15)
+        XCTAssertEqual(FlowCaptureTailDrainPolicy.flowDefault, FlowCaptureTailDrainPolicy.iosFlow)
+    }
+}