Fix vacuous bounds test and stale/circular comments from the settle

Replace the self-comparing render-window loop with a genuinely discriminating
floor-vs-exact check; correct two stale claims; mark the Auto/Manual floor-parity
premise as unverified; drop the STARTPOS/ENDPOS comment's circular justification.

src/shell/capture/capture.cpp

@@ -464,11 +464,9 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
     // millisecond (render_settings.h's kRenderBoundsTimeSelection).
     //
     // RENDER_STARTPOS/ENDPOS are written anyway, to the same window. The header
-    // (~3045-3046) documents them as mode-0-only, but the DAW run that settled this
-    // channel had both stores holding the identical window, so it cannot distinguish
-    // "mode 2 ignored them" from "mode 2 read them and they happened to agree". Writing
-    // them keeps the two stores agreeing rather than resting exactness on that
-    // distinction; a stale leftover here could only ever misalign a render silently.
+    // (~3045-3046) documents them as mode-0-only, so on mode 2 this is a cheap,
+    // fully-restored (ScopedRenderSettings) defensive write against that
+    // documentation being an incomplete account of what the renderer reads.
     GetSetProjectInfo(proj, "RENDER_BOUNDSFLAG",
                       static_cast<double>(kRenderBoundsTimeSelection), true);
     GetSetProjectInfo(proj, "RENDER_STARTPOS", request.startSeconds, true);