test(engine): extract ShouldAutoRebuild as pure fn + cover 6 cases

The self-heal trigger from c30a616 was time-based logic embedded in the RunAsync poll loop — easy to regress on a future refactor without anyone noticing. Pull it out into a public static ShouldAutoRebuild(sinceStart, sinceLastSeen, sinceLastRebuild) that returns the rebuild reason or null. RunAsync just calls it and acts on the result.

Six new test cases cover the matrix:
  - never seen + before warmup       -> hold
  - never seen + after warmup        -> rebuild
  - never seen + recent rebuild      -> backoff
  - had sources + long-gone          -> rebuild
  - had sources + recently gone      -> grace window
  - had sources + recent rebuild     -> backoff

112/112 Engine tests passing (was 106; +6 new).

src/TeamsISO.Engine/Discovery/NdiDiscoveryService.cs

@@ -127,36 +127,19 @@ public sealed class NdiDiscoveryService
                     RebuildFinder("operator request");
                     lastRebuildAt = now;
                 }
-                // Auto-healing rebuilds. Only one path can fire per tick.
+                // Auto-healing rebuilds — see ShouldAutoRebuild.
                 else if (_previous.Count == 0)
                 {
-                    var sinceStart = now - startedAt;
-                    var sinceRebuild = now - lastRebuildAt;
-                    if (lastSeenAt is null)
+                    var decision = ShouldAutoRebuild(
+                        sinceStart:      now - startedAt,
+                        sinceLastSeen:   lastSeenAt is { } seen ? now - seen : (TimeSpan?)null,
+                        sinceLastRebuild: now - lastRebuildAt);
+                    if (decision is { } reason)
                     {
-                        // Never seen a source. After 5s of empty results, try a rebuild
-                        // every 5s. This handles the cold-start case where the initial
-                        // finder didn't bind to the right network interface.
-                        if (sinceStart > TimeSpan.FromSeconds(5) && sinceRebuild > TimeSpan.FromSeconds(5))
-                        {
-                            RebuildFinder("auto-heal: never saw a source");
+                        RebuildFinder(reason);
                         lastRebuildAt = now;
                     }
                 }
-                    else
-                    {
-                        // We've seen sources before but currently see nothing. After
-                        // 15s of empty results, try a rebuild every 10s. This handles
-                        // the "Teams stopped broadcasting then started again but our
-                        // finder didn't pick up the new advertisements" case.
-                        var sinceLastSeen = now - lastSeenAt.Value;
-                        if (sinceLastSeen > TimeSpan.FromSeconds(15) && sinceRebuild > TimeSpan.FromSeconds(10))
-                        {
-                            RebuildFinder("auto-heal: source set went empty 15s ago");
-                            lastRebuildAt = now;
-                        }
-                    }
-                }
 
                 try { PollOnce(); }
                 catch (Exception ex) { _logger.LogWarning(ex, "Discovery poll failed; will retry on next tick."); }
@@ -170,6 +153,39 @@ public sealed class NdiDiscoveryService
         }
     }
 
+    /// <summary>
+    /// Pure-function decision for whether the discovery loop should rebuild the
+    /// NDI finder on the current tick. Returns a non-null reason string when
+    /// the rebuild should fire (which is also logged); null means "leave the
+    /// finder alone." Caller is responsible for tracking the timestamps and
+    /// updating <c>lastRebuildAt</c> after the rebuild.
+    ///
+    /// Public + static for unit-testability — the time-based rules are easy to
+    /// regress and hard to spot in integration testing.
+    ///
+    /// Rules:
+    /// <list type="number">
+    ///   <item><b>Never seen a source</b> (<paramref name="sinceLastSeen"/> is null):
+    ///         rebuild when sinceStart &gt; 5s AND sinceLastRebuild &gt; 5s.</item>
+    ///   <item><b>Used to see sources, now empty</b>: rebuild when sinceLastSeen
+    ///         &gt; 15s AND sinceLastRebuild &gt; 10s.</item>
+    /// </list>
+    /// Both rules back off the rebuild cadence to avoid churn during legitimate
+    /// empty periods (no meeting active, all participants left, etc.).
+    /// </summary>
+    public static string? ShouldAutoRebuild(TimeSpan sinceStart, TimeSpan? sinceLastSeen, TimeSpan sinceLastRebuild)
+    {
+        if (sinceLastSeen is null)
+        {
+            if (sinceStart > TimeSpan.FromSeconds(5) && sinceLastRebuild > TimeSpan.FromSeconds(5))
+                return "auto-heal: never saw a source";
+            return null;
+        }
+        if (sinceLastSeen.Value > TimeSpan.FromSeconds(15) && sinceLastRebuild > TimeSpan.FromSeconds(10))
+            return "auto-heal: source set went empty 15s ago";
+        return null;
+    }
+
     /// <summary>
     /// Dispose the current finder and create a fresh one against the cached
     /// discovery groups. Clears the seen-set so all currently-visible sources

src/tests/TeamsISO.Engine.Tests/Discovery/NdiDiscoveryServiceTests.cs

@@ -66,4 +66,78 @@ public class NdiDiscoveryServiceTests
         while (reader.TryRead(out var ev)) list.Add(ev);
         return list;
     }
+
+    // ============================================================
+    // ShouldAutoRebuild — pure function gating the auto-heal path
+    // ============================================================
+    //
+    // Two rules under test:
+    //   (a) Never seen a source AND sinceStart>5s AND sinceLastRebuild>5s -> rebuild
+    //   (b) Used to see sources, now empty AND sinceLastSeen>15s AND sinceLastRebuild>10s -> rebuild
+    //
+    // Both rules back off to avoid churn during legitimate empty periods.
+
+    [Fact]
+    public void ShouldAutoRebuild_NeverSeenSource_BeforeWarmup_ReturnsNull()
+    {
+        // 3s after startup is well inside the "give cold start a chance" window.
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromSeconds(3),
+            sinceLastSeen: null,
+            sinceLastRebuild: TimeSpan.FromSeconds(99))
+            .Should().BeNull();
+    }
+
+    [Fact]
+    public void ShouldAutoRebuild_NeverSeenSource_AfterWarmup_TriggersRebuild()
+    {
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromSeconds(6),
+            sinceLastSeen: null,
+            sinceLastRebuild: TimeSpan.FromSeconds(6))
+            .Should().Contain("never saw a source");
+    }
+
+    [Fact]
+    public void ShouldAutoRebuild_NeverSeenSource_RecentRebuild_HoldsOff()
+    {
+        // sinceStart qualifies, but the last rebuild was 2s ago — back off.
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromSeconds(20),
+            sinceLastSeen: null,
+            sinceLastRebuild: TimeSpan.FromSeconds(2))
+            .Should().BeNull();
+    }
+
+    [Fact]
+    public void ShouldAutoRebuild_HadSources_NowEmpty_LongAgo_TriggersRebuild()
+    {
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromMinutes(5),
+            sinceLastSeen: TimeSpan.FromSeconds(20),
+            sinceLastRebuild: TimeSpan.FromSeconds(30))
+            .Should().Contain("source set went empty");
+    }
+
+    [Fact]
+    public void ShouldAutoRebuild_HadSources_NowEmpty_Recently_HoldsOff()
+    {
+        // 10s since last source seen — still inside the 15s grace window.
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromMinutes(5),
+            sinceLastSeen: TimeSpan.FromSeconds(10),
+            sinceLastRebuild: TimeSpan.FromSeconds(30))
+            .Should().BeNull();
+    }
+
+    [Fact]
+    public void ShouldAutoRebuild_HadSources_NowEmpty_RecentRebuild_HoldsOff()
+    {
+        // Grace window expired, but we just rebuilt 8s ago — back off.
+        NdiDiscoveryService.ShouldAutoRebuild(
+            sinceStart: TimeSpan.FromMinutes(5),
+            sinceLastSeen: TimeSpan.FromSeconds(30),
+            sinceLastRebuild: TimeSpan.FromSeconds(8))
+            .Should().BeNull();
+    }
 }