Settle the render window on the time selection and delete the experiment that proved it

The millisecond floor lives in the custom-bounds field, not the engine, so
RENDER_BOUNDSFLAG=2 is now the only bounds mode: the two-position type, the
console verdict and the STARTPOS/ENDPOS drift probe all go. capture.cpp 697 -> 622.

src/shell/capture/capture_orchestrator.cpp

@@ -231,8 +231,8 @@ CaptureResult renderOffline(CaptureScope scope,
 // caller can report success/failure. Load-bearing principle holds: writes a file +
 // a bank index entry ONLY; never touches the arrange/timeline. Non-destructive: the
 // out-of-scope FX/fader/pan chain is fully restored on every path (FxBypassGuard),
-// and the backend restores every RENDER_* setting it changed plus, on the
-// time-selection bounds channel, the project time selection it borrowed.
+// and the backend restores every RENDER_* setting it changed plus the project time
+// selection it borrowed to carry the render window.
 //
 // On success, res.sample.id carries the LANDED bank-index id (S8): the newly-added id
 // on a fresh add, or the EXISTING entry's id on a hash-dedup collapse — so the S8