⚡ Bolt: Avoid HashMap allocation on cancellation hot path

[ovasylenko]

💡 What: Replaced a HashMap that maps parent nodes to children with a sorted Vec<(ExecutionNodeId, &ExecutionNode)> in orch8-engine/src/signals.rs.
🎯 Why: HashMap allocation and subsequent dynamic .entry().push() calls introduce severe hashing and memory allocation overhead on the cancel_scoped execution hot path.
📊 Impact: Reduces memory allocation and hashing overhead across the cancellation hot path, resulting in faster and lighter tree node traversals, particularly when evaluating deeply nested configurations.
🔬 Measurement: Verified by running cargo test -p orch8-engine --lib signals and executing the existing tests correctly.

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PR created automatically by Jules for task 14150581645333128300 started by @ovasylenko

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