Prove the render bounds at the boundary they cross, and name a short render whose count is exactly a millisecond-floored window

No truncation exists on our side of that boundary, so the read-back is the
only evidence available for whether REAPER kept the window — and it fires on
every tail mode, where only None was ever judged.

src/shell/capture/render_bounds_gate.cpp

@@ -99,6 +99,17 @@ BoundsVerdict checkRenderedBounds(const std::string& renderedPath,
     const long long actualFrames = static_cast<long long>(layout.frameCount());
     if (renderHonoredBounds(expectedFrames, actualFrames)) return v;
 
+    // Says whether this shortfall has the one shape two live short renders already
+    // matched to the frame, so every refusal from here on adds to (or breaks) that
+    // evidence instead of needing the arithmetic done by hand. A count coincidence
+    // only — it does not establish how the render resolved anything.
+    const std::string msNote =
+        actualFrames == msFlooredEndFrameCount(request.startSeconds,
+                                               request.endSeconds, rate)
+            ? " Those are exactly the frames this window holds with its end floored to"
+              " the millisecond -- a match on the count, not a measured cause."
+            : std::string();
+
     v.refused = true;
     v.message = "Render produced " + std::to_string(actualFrames) +
                 " frames but the requested range is " + std::to_string(expectedFrames) +
@@ -108,7 +119,7 @@ BoundsVerdict checkRenderedBounds(const std::string& renderedPath,
                 std::to_string(request.endSeconds) + "s) -> frame indices [" +
                 std::to_string(std::llround(request.startSeconds * rate)) + ", " +
                 std::to_string(std::llround(request.endSeconds * rate)) + ")." +
-                retainRefusedRender(renderedPath, projectDir);
+                msNote + retainRefusedRender(renderedPath, projectDir);
     return v;
 }