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

src/shell/capture/capture_realtime_shell.cpp

@@ -30,8 +30,9 @@
 // TAP: the hidden temp track receives a send FROM each selected source track
 // (CreateTrackSend(source, temp)) and records its own output (B_MAINSEND=0, so
 // it never sums back into the master — no feedback, no monitoring double).
-// Multiple selected tracks sum in the one temp track, matching how offline
-// track scope handles a multi-track selection.
+// Multiple selected tracks sum in the one temp track — a real mix, which is why
+// realtime accepts a multi-track selection where the offline track scope refuses
+// it (that render source cannot express a sum; see shell/capture/CLAUDE.md).
 //
 // Why this needs no FxBypassGuard: CreateTrackSend defaults to I_SENDMODE=0
 // (post-fader), which taps the source track after its own FX/fader/pan — its