hip: add controllable 50% DG lesion during AC training

[Copilot]

Adds a DGLesion toggle to the hippocampus sim (ch7/hip) that lesions 50% of DG (dentate gyrus) neurons before AC list training and restores the layer at the start of each new AB run.

Changes

• Sim.DGLesion bool — new exported field; automatically surfaces as a checkbox in the Cogent Core control panel with no extra widget wiring
• AB→AC transition (TestAtInterval) — when the training env flips from TrainAB to TrainAC, calls LesionNeurons(0.5) on DG if the flag is set
• NewRun() — unconditionally calls UnLesionNeurons() on DG before InitWeights(), so every AB training run starts with a fully intact DG regardless of the previous run's lesion state

// in TestAtInterval, at the AB→AC switch point:
if ss.DGLesion {
    ss.Net.LayerByName("DG").LesionNeurons(0.5)
}

// in NewRun(), before InitWeights:
ss.Net.LayerByName("DG").UnLesionNeurons()

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