fix(renderer): render step timeline once after all charts

[Copilot]

Step timeline was rendered inside the chart loop, appearing twice in the summary (once after CPU Loads, once after Memory Usages).

Changes

• Moved step annotation generation outside chart iteration loop
• Step timeline now renders once after all charts
• Conditionally generate annotations only when charts contain data

Structure

Before:

### CPU Loads
[chart]
#### Step Timeline
...

### Memory Usages  
[chart]
#### Step Timeline  ← duplicate
...

After:

### CPU Loads
[chart]

### Memory Usages
[chart]

#### Step Timeline  ← single occurrence
...

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