V13-FE-011: finalize search list layout slice

docs/Design/kbx-foundation-v36/backend/Shared/Experiments/KbxExperimentAssignmentService.cs

@@ -0,0 +1,45 @@
+using Dapper;
+using Kbx.Shared.Experiments.Generated;
+using Npgsql;
+
+namespace Kbx.Shared.Experiments;
+
+public sealed class KbxExperimentAssignmentService(NpgsqlDataSource dataSource)
+{
+    public async Task<KbxExperimentAssignmentsResponse> EvaluateAsync(Guid tenantId,Guid userId,string screenId,CancellationToken ct)
+    {
+        var definitions=KbxExperimentCatalog.Experiments.Values.Where(x=>x.ScreenId==screenId).ToArray();
+        var items=new List<KbxExperimentAssignmentDto>();
+        await using var connection=await dataSource.OpenConnectionAsync(ct);
+        foreach(var definition in definitions)
+        {
+            var runtime=await connection.QuerySingleOrDefaultAsync<RuntimeRow>(new CommandDefinition("""
+                select state State,rollout_percent RolloutPercent,kill_switch KillSwitch
+                  from kbx.experiment_runtime where tenant_id=@TenantId and experiment_id=@ExperimentId;
+            """,new{TenantId=tenantId,ExperimentId=definition.Id},cancellationToken:ct));
+            var state=runtime?.State ?? definition.State;
+            var rollout=runtime?.RolloutPercent ?? definition.RolloutPercent;
+            var killed=runtime?.KillSwitch ?? false;
+            // Regular users only receive actively enrolled assignments. Draft/paused/rolled-back definitions stay internal.
+            if(killed||state!="running"||rollout<=0)continue;
+            // Rollout percentage remains authoritative even for previously assigned users.
+            // Assignment is preserved for reproducibility and becomes active again if rollout is raised later.
+            if(!KbxExperimentAssignmentPolicy.IsEnrolled(tenantId,userId,definition.Id,rollout))continue;
+            var existing=await connection.QuerySingleOrDefaultAsync<string>(new CommandDefinition("""
+                select variant from kbx.experiment_assignments where tenant_id=@TenantId and experiment_id=@ExperimentId and user_id=@UserId;
+            """,new{TenantId=tenantId,ExperimentId=definition.Id,UserId=userId},cancellationToken:ct));
+            if(existing is not null){items.Add(new(definition.Id,definition.FlagId,screenId,true,existing,state));continue;}
+            var variant=KbxExperimentAssignmentPolicy.ChooseVariant(tenantId,userId,definition);
+            await connection.ExecuteAsync(new CommandDefinition("""
+                insert into kbx.experiment_assignments(tenant_id,experiment_id,user_id,variant)
+                values(@TenantId,@ExperimentId,@UserId,@Variant) on conflict do nothing;
+            """,new{TenantId=tenantId,ExperimentId=definition.Id,UserId=userId,Variant=variant},cancellationToken:ct));
+            existing=await connection.QuerySingleAsync<string>(new CommandDefinition("""
+                select variant from kbx.experiment_assignments where tenant_id=@TenantId and experiment_id=@ExperimentId and user_id=@UserId;
+            """,new{TenantId=tenantId,ExperimentId=definition.Id,UserId=userId},cancellationToken:ct));
+            items.Add(new(definition.Id,definition.FlagId,screenId,true,existing,state));
+        }
+        return new(items);
+    }
+    private sealed record RuntimeRow(string State,int RolloutPercent,bool KillSwitch);
+}