feat(cms): dedicated grid columns for medium-specific row actions

Replace CmsAlbumBrowser's single RowActions slot with a SpecialColumns
list (header + per-row cell). Mix renders a Waveform column, Session a
Hero column, between Tracks and Actions; Edit/Delete stay rightmost.
Child-row colspan now computed from column count. Cut/ALL unchanged.

DeepDrftManager/Components/Pages/Tracks/SpecialActionColumn.cs

@@ -0,0 +1,15 @@
+using DeepDrftModels.DTOs;
+using Microsoft.AspNetCore.Components;
+
+namespace DeepDrftManager.Components.Pages.Tracks;
+
+/// <summary>
+/// A dedicated, header-labelled grid column for a medium-specific row affordance (e.g. Mix waveform
+/// generate, Session hero upload) in <see cref="CmsAlbumBrowser"/>. A per-medium host declares zero or
+/// more of these; the grid renders one extra header cell and one extra per-row cell for each, positioned
+/// between the Tracks column and the universal Actions (Edit/Delete) column. The <see cref="Cell"/>
+/// fragment is handed each release; the host recovers its typed row state via its own RowFor lookup.
+/// </summary>
+/// <param name="Header">Column header label (e.g. "Waveform", "Hero").</param>
+/// <param name="Cell">Per-row cell content for a given release.</param>
+public sealed record SpecialActionColumn(string Header, RenderFragment<ReleaseDto> Cell);