Fix render-bounds EXACT verdict: enumerate floored models instead of trusting grid membership

Grid-ness of an edge was a proxy for "no floor could explain this count," not
the test itself — equal remainders on both edges cancel under a full floor. Now
checks all three floored models directly and corrects the SHORT/LONG
floor-signature docs.

src/shell/capture/capture.cpp

@@ -563,10 +563,11 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
     // because a verdict that appeared only on some outcomes would read its own absence
     // on the rest as a pass. Auto/Manual are not judged against a frame count (they add
     // frames by design) and the sentence says so rather than comparing anyway.
-    // SelectedItems/RazorArea derive their bounds from content and never consult the
-    // channel at all (render_settings::sourceBypassesBoundsChannel) — the batch-item
-    // path renders through SelectedItems on every capture, so without this the verdict
-    // would print an EXACT/SHORT/LONG claim about a channel that was never in play.
+    // SelectedItems/RazorArea are INFERRED (not SDK-confirmed) to derive their bounds
+    // from content and never consult the channel at all
+    // (render_settings::sourceBypassesBoundsChannel) — the batch-item path renders
+    // through SelectedItems on every capture, so without this the verdict would print an
+    // EXACT/SHORT/LONG claim about a channel that was never in play.
     const char* boundsBypassLabel = sourceBypassesBoundsChannel(request.sourceMode)
         ? renderSourceLabel(request.sourceMode)
         : nullptr;