Task: Read and write the market series asset classes in Qt

The Qt client reads and writes the market_series_asset_classes junction, so the market series grid shows the asset classes each series belongs to and the New Series form records them.

• A protocol carries the asset classes for each market series, hand-written because junction messaging is not generated.
• The client fetches the junction rows with the series and joins them in memory.
• The market series grid shows each class's catalogue name and filters on it.
• The New Series form writes one junction row per class the series key yields.
• A series created through the form has junction rows; today it has none.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

This task restores something, it does not only add it. The junction change removed three things from the UI, because the data behind them left the series row and no read path replaced it:

• The market series grid's Asset Class column, which showed the raw asset_class code.
• The grid's asset class filter combo box on the toolbar, which filtered that column client-side.
• The New Series form's asset class combo box, so a series created through the form now has no junction rows at all.

Each comes back here, sourced from the junction rather than the series row. The grid column and the filter should read the catalogue name, which is the display correction PR #2092 recorded and could not make.

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Abandoned at sprint 25 close. The Qt client was removed, and the codegen work goes to the sprint 26 per-module codegen pass.