feat(tui): live GPU chart during stress test, full VRAM allocation

- GPU Platform Stress Test now shows a live in-TUI chart instead of nvtop.
  nvidia-smi is polled every second; up to 60 data points per GPU kept.
  All three metrics (Usage %, Temp °C, Power W) drawn on a single plot,
  each normalised to its own range and rendered in a different colour.
- Memory allocation changed from MemoryMB/16 to MemoryMB-512 (full VRAM
  minus 512 MB driver overhead) so bee-gpu-stress actually stresses memory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/tui/messages.go

@@ -50,3 +50,8 @@ type gpuStressDoneMsg struct {
 	body  string
 	err   error
 }
+
+type gpuLiveTickMsg struct {
+	rows    []platform.GPUMetricRow
+	indices []int
+}