Refuse every multi-track selected-tracks render, both scopes; make a failed mono collapse observable

src/shell/capture/CLAUDE.md

@@ -74,11 +74,17 @@ detail not covered there:
   each other (the former `ICaptureBackend` was removed) — do not reintroduce one
   without a real second polymorphic call site.
 - **The selected-tracks render (`&128`) is read as emitting one file per selected
-  track** — the single-file bit is documented for item/razor sources only (SDK header
-  ~3041), and that is the whole basis for the reading; it is DAW-unverified. If it
-  holds, then since `RENDER_PATTERN` is one literal stem and success is a file-exists
-  check, N tracks would land one track's audio as a successful capture.
-  `renderOffline` refuses that shape for the RANGED ITEM capture only
-  (`render_settings::isMultiTrackRangedItemRender`). Track scope renders through the
-  same source with the same exposure and is deliberately untouched here — filed in
-  `docs/TODO.md`.
+  track** — the single-file bit `&(4<<16)` is documented for item/razor sources only
+  (SDK header ~3041), and that is the whole basis for the reading; it is DAW-unverified.
+  If it holds, then since `RENDER_PATTERN` is one literal stem and success is a
+  file-exists check, N tracks would land one track's audio as a successful capture.
+  `renderOffline` refuses EVERY multi-track render through that source
+  (`render_settings::isMultiTrackStemRender`) — the ranged item capture and the plain
+  track capture alike, each with its own way out
+  (`render_settings::multiTrackRefusalMessage`). The refusal is keyed on the render
+  SOURCE and not on the scope, so a future caller that reaches `&128` inherits it.
+  Re-opening a multi-track track capture needs the DAW check in
+  `docs/verify-track-scope-multitrack.md` to come back the other way first.
+- **Realtime is the one capture path that accepts a multi-track selection**, and it is
+  correct to: its per-source-track sends sum in the one temp track, which is a real mix
+  rather than a stem collapse. The offline refusal above does not apply to it.