Implement result caching with dirty propagation

[Konstantysz]

Problem

Changing a single parameter (e.g., threshold value) re-executes the entire graph, including unchanged upstream nodes. This is wasteful for 10+ node pipelines.

Proposal

Cache node outputs and only re-execute dirty nodes.

Design

• Add Node::m_isDirty flag and Node::MarkDirty()
• Propagate dirty flag downstream when:
  • Node parameter changes
  • Input connection changes
  • Upstream node is marked dirty
• Cache last output in Node::m_cachedOutput
• Skip Process() if !m_isDirty and cache exists

Memory Management

Add cache size limit (e.g., 500MB), LRU eviction.

Acceptance Criteria

• Graph with 10 nodes: changing last node param only re-executes last node
• Cache invalidates correctly when connections change
• Memory usage stays below limit