Fix solution edges dropped during GEFF export when not in LinkDB

[ThibautGoldsborough]

Bug

to_geff_from_database silently drops solution edges that exist in NodeDB.parent_id but have no corresponding entry in the LinkDB (links) table.

This happens because the export collects solution edges from NodeDB.parent_id, then merges them onto the LinkDB edges using a left join on (source_id, target_id). Since the left join is anchored on LinkDB, any solution edge without a matching link row is discarded.

When it fails

This affects any workflow where an annotator manually creates a parent-child connection that wasn't in the original candidate edge set. The most common case is cell divisions: when an annotator connects a mother cell to a second daughter, this new edge exists only in NodeDB.parent_id and not in LinkDB. The exported GEFF file will be missing that edge, so the division is lost or appears as a regular track.

Fix

Change the merge from how="left" to how="outer" so that solution edges not present in LinkDB are preserved in the output. The weight column is filled with 0.0 for these added edges since they have no candidate link weight.

# Before
edge_df = edge_df.merge(sol_links_df, on=["source_id", "target_id"], how="left")
...
edge_df["weight"] = edge_df["weight"].astype(np.float64)

# After
edge_df = edge_df.merge(sol_links_df, on=["source_id", "target_id"], how="outer")
...
edge_df["weight"] = edge_df["weight"].fillna(0.0).astype(np.float64)

[codecov]

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