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>

src/AcDream.App/Diagnostics/FrameStatsBuffer.cs

@@ -56,8 +56,13 @@ public sealed class FrameStatsBuffer
 
     public long Max()
     {
-        long max = 0;
-        for (int i = 0; i < _count; i++)
+        if (_count == 0) return 0;   // documented empty behavior, matches Percentile
+        // Seed from the first live sample so all-negative windows (the alloc
+        // channel can go negative if the boundary ever crosses threads) return
+        // the true max instead of clamping to 0. Slots [0.._count) are always
+        // the live window regardless of ring wraparound.
+        long max = _samples[0];
+        for (int i = 1; i < _count; i++)
             if (_samples[i] > max) max = _samples[i];
         return max;
     }