fix(pipeline): MP0 - profiler toggle-path hygiene + Max() seed (review follow-up)

Three review finds, all in the runtime-toggle path or edge math (the
steady-state path was confirmed correct):

1. GPU stale-slot across toggle-off/on: the disabled branch now
   Disposes the GpuFrameTimer (and nulls it) instead of Stop()-ing and
   keeping it — a kept instance would poll slots left pending from
   BEFORE the pause on re-enable and report temporally stale GPU
   samples. The re-enable branch's existing null guard rebuilds the
   ring fresh. Dispose is safe there: the branch runs at the top of
   OnRender with the GL context current.

2. Stale stage accumulation across mid-stage toggle-off: a StageScope
   disposed after the flag flips still calls EndStage, leaving a
   partial delta in _stageAccumTicks. The re-baseline branch now
   Array.Clear()s it alongside the GC re-baselining.

3. FrameStatsBuffer.Max() seeded with 0, clamping all-negative windows
   (the alloc channel can go negative if the boundary ever crosses
   threads) and disagreeing with Percentile(). Now seeds from the
   first live sample after the empty check (slots [0.._count) are
   always the live window regardless of ring wraparound); empty still
   returns 0 to match Percentile. TDD: Max_AllNegative_ReturnsTrueMax
   failed (returned 0) before the fix, passes after.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/FrameStatsBufferTests.cs

@@ -48,4 +48,12 @@ public class FrameStatsBufferTests
         Assert.Equal(0, buf.Count);
         Assert.Equal(0, buf.Percentile(0.5));
     }
+
+    [Fact]
+    public void Max_AllNegative_ReturnsTrueMax()
+    {
+        var buf = new FrameStatsBuffer(capacity: 4);
+        buf.Push(-5); buf.Push(-2); buf.Push(-9);
+        Assert.Equal(-2, buf.Max());
+    }
 }