Ψ-W3 remediation: fix the verify doc's wrong render source, add missing content checks, soften unverified claims

Corrected the Render-dialog source name the refusal's evidence depends on, added a by-ear content check and a file-size channel proxy, and stopped two comments from overclaiming.

src/shell/capture/capture_realtime_shell.cpp

@@ -32,7 +32,8 @@
 // it never sums back into the master — no feedback, no monitoring double).
 // 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).
+// it (the offline render source is read as emitting one file per track, DAW-
+// unverified — see src/shell/capture/CLAUDE.md §Gotchas).
 //
 // 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