fix: rename BondRef.ProtoID to SourceID for clarity (bd-ia3g)

The ProtoID field in BondRef was misleading as it could hold both proto
IDs (from proto+proto bonds) and molecule IDs (from mol+mol bonds).
Rename to SourceID with updated JSON tag to better reflect its purpose.

Changes:
- Rename BondRef.ProtoID to SourceID in types.go
- Update JSON tag from proto_id to source_id
- Update all usages in mol_bond.go and tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/mol_bond.go

@@ -317,8 +317,8 @@ func bondProtoProto(ctx context.Context, s storage.Storage, protoA, protoB *type
 			Priority:    minPriority(protoA.Priority, protoB.Priority),
 			IssueType:   types.TypeEpic,
 			BondedFrom: []types.BondRef{
-				{ProtoID: protoA.ID, BondType: bondType, BondPoint: ""},
-				{ProtoID: protoB.ID, BondType: bondType, BondPoint: ""},
+				{SourceID: protoA.ID, BondType: bondType, BondPoint: ""},
+				{SourceID: protoB.ID, BondType: bondType, BondPoint: ""},
 			},
 		}
 		if err := tx.CreateIssue(ctx, compound, actorName); err != nil {