capture: name the render source in the exact-bounds refusal, and put its one-frame tolerance under test

The tolerance is unchanged and now derived, not assumed: frameCountFor lands in {floor(L), ceil(L)}, so a non-frame-aligned window can never miss by more than a frame. Naming the source is what tells a self-bounding render from a short one.

src/core/capture/render_window.h

@@ -18,6 +18,18 @@ namespace reasampler::capture {
 // whether the equality is exact or off by a frame.
 long long frameCountFor(double startSeconds, double endSeconds, int sampleRate);
 
+// True when a landed render's frame count is consistent with `frameCountFor`'s
+// answer for the same window. Tolerates a one-frame difference, and exactly one:
+// frameCountFor rounds EACH edge, so it sits within a frame of the window's
+// real-valued length (end-start)*rate — and a renderer that floors, ceils or
+// rounds that same length sits within a frame of it too, so two integers derived
+// that way can never be more than one apart. A window whose edges do not land on
+// frame boundaries therefore cannot produce a larger difference; anything larger
+// is a render that printed something other than the window asked for, whatever
+// the alignment. Widening this past one frame retires the exact-bounds invariant
+// rather than relaxing it — do not.
+bool renderHonoredBounds(long long expectedFrames, long long actualFrames);
+
 // True when a render bounded by the selected items' own extent
 // [itemStart, itemEnd) already prints exactly the requested
 // [reqStart, reqEnd) window — the one case where REAPER's selected-items render