Add intra-population edges to test circuit, fix external edge merge bug

[cattabiani]

Summary

Adds intra-population edges (A→A, B→B) to the split_subcircuit test circuit and fixes a bug in _copy_filtered_edges exposed by the new edge topology.

Test data changes

• A→A edges: A3→A4, A4→A3, A1→A2, A0→A4, A5→A1
• B→B edges: B0→B2, B3→B4
• C→C: intentionally omitted to test the case where no intra-population edge file exists

Updated circuit_config.json, circuit_config_subcircuit.json, and regenerated edges.h5. All test assertions updated to reflect the new external populations and edge counts.

Bug fix: source ID lookup in multi-source external populations

The problem

_copy_filtered_edges used to concatenate all source DataFrames into one:

src_concat = pd.concat(id_mapping.data[write_edge_config.src_mapping].values())

This is conceptually wrong. The sources come from different ID spaces (e.g., the parent's external_A population vs the local A population). When two sources happen to share the same local node ID, the concatenated DataFrame has duplicate index values, and .loc[id] returns multiple rows — corrupting the edge file.

Concrete example

When extracting c4 from c3_c2_c1, the external_A population has two sources:

• "external_A": node 2 (inherited from parent) → new_id 0
• "A": node 2 (newly removed) → new_id 1

The concat produces index [2, 2]. When processing the parent's external_A__C edge (src=2, tgt=3), the lookup src_concat.loc[2] returns both rows, appending [0, 1] to source_node_id but only [3] to target_node_id.

The fix

Removed src_concat entirely. Each input now resolves its own source DataFrame directly via source_filter. For single-source populations (the common case), the single entry is used directly. For multi-source populations, an assertion enforces that source_filter is always provided.

Why it wasn't triggered before

The bug requires intra-population edges (A→A) so that removed A nodes become external_A via the same-population path. Without them, external nodes were only discovered through inter-population edges, and the local ID collision never occurred.