Phase 3C: Deployment pipeline & Store-and-Forward engine

Deployment Manager (WP-1–8, WP-16):
- DeploymentService: full pipeline (flatten→validate→send→track→audit)
- OperationLockManager: per-instance concurrency control
- StateTransitionValidator: Enabled/Disabled/NotDeployed transition matrix
- ArtifactDeploymentService: broadcast to all sites with per-site results
- Deployment identity (GUID + revision hash), idempotency, staleness detection
- Instance lifecycle commands (disable/enable/delete) with deduplication

Store-and-Forward (WP-9–15):
- StoreAndForwardStorage: SQLite persistence, 3 categories, no max buffer
- StoreAndForwardService: fixed-interval retry, transient-only buffering, parking
- ReplicationService: async best-effort to standby (fire-and-forget)
- Parked message management (query/retry/discard from central)
- Messages survive instance deletion, S&F drains on disable

620 tests pass (+79 new), zero warnings.

tests/ScadaLink.DeploymentManager.Tests/OperationLockManagerTests.cs

@@ -0,0 +1,95 @@
+namespace ScadaLink.DeploymentManager.Tests;
+
+/// <summary>
+/// WP-3: Tests for per-instance operation lock.
+/// </summary>
+public class OperationLockManagerTests
+{
+    private readonly OperationLockManager _lockManager = new();
+
+    [Fact]
+    public async Task AcquireAsync_ReturnsDisposable()
+    {
+        using var lockHandle = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        Assert.NotNull(lockHandle);
+    }
+
+    [Fact]
+    public async Task AcquireAsync_SameInstance_BlocksSecondCaller()
+    {
+        using var firstLock = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+
+        // Second acquire should time out
+        await Assert.ThrowsAsync<TimeoutException>(() =>
+            _lockManager.AcquireAsync("inst1", TimeSpan.FromMilliseconds(50)));
+    }
+
+    [Fact]
+    public async Task AcquireAsync_DifferentInstances_BothSucceed()
+    {
+        using var lock1 = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        using var lock2 = await _lockManager.AcquireAsync("inst2", TimeSpan.FromSeconds(5));
+
+        Assert.NotNull(lock1);
+        Assert.NotNull(lock2);
+    }
+
+    [Fact]
+    public async Task AcquireAsync_AfterRelease_CanReacquire()
+    {
+        var firstLock = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        firstLock.Dispose();
+
+        // Should succeed now
+        using var secondLock = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        Assert.NotNull(secondLock);
+    }
+
+    [Fact]
+    public async Task IsLocked_ReturnsTrueWhileLocked()
+    {
+        Assert.False(_lockManager.IsLocked("inst1"));
+
+        using var lockHandle = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        Assert.True(_lockManager.IsLocked("inst1"));
+    }
+
+    [Fact]
+    public async Task IsLocked_ReturnsFalseAfterRelease()
+    {
+        var lockHandle = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        lockHandle.Dispose();
+
+        Assert.False(_lockManager.IsLocked("inst1"));
+    }
+
+    [Fact]
+    public async Task AcquireAsync_DoubleDispose_DoesNotThrow()
+    {
+        var lockHandle = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(5));
+        lockHandle.Dispose();
+        lockHandle.Dispose(); // Should not throw
+    }
+
+    [Fact]
+    public async Task AcquireAsync_CancellationToken_Respected()
+    {
+        using var firstLock = await _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(30));
+
+        using var cts = new CancellationTokenSource(TimeSpan.FromMilliseconds(50));
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(() =>
+            _lockManager.AcquireAsync("inst1", TimeSpan.FromSeconds(30), cts.Token));
+    }
+
+    [Fact]
+    public async Task AcquireAsync_ConcurrentDifferentInstances_AllSucceed()
+    {
+        var tasks = Enumerable.Range(0, 10).Select(async i =>
+        {
+            using var lockHandle = await _lockManager.AcquireAsync($"inst{i}", TimeSpan.FromSeconds(5));
+            await Task.Delay(10);
+        });
+
+        await Task.WhenAll(tasks);
+    }
+}