fix(queue): prevent usize underflow in frame_queue prune when queue is empty

[lavidaloca26]

Problem

The FrameQueue::prune() function has an off-by-one error that causes a usize underflow panic. When the queue has fewer than 2 elements, the expression self.queue.len() - 1 underflows because usize is unsigned in Rust.

Fix

Added an early return check if self.queue.len() < 2 { return; } before the while loop to prevent the underflow. This is safe because the pruning logic requires at least 2 frames to compare.

Why change is safe

• Early return preserves existing behavior for empty or single-frame queues (nothing to prune)
• The pruning logic only operates on consecutive frame pairs, requiring 2+ frames
• No breaking changes to API or external behavior
• Defensive fix that prevents panic while maintaining correctness

[cb-heimdall]

🟡 Heimdall Review Status

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