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/core/capture/render_window.h

@@ -1,9 +1,12 @@
 #pragma once
 // render_window — pure frame arithmetic for a capture's requested window: the
-// frame count a project-time range occupies, and whether a render whose bounds
-// come from the selected items' own extent already prints that window.
+// frame count a project-time range occupies, whether a render whose bounds come
+// from the selected items' own extent already prints that window, and the two
+// diagnostics that say where a short render lost its frames.
 // NO REAPER types; unit-tested by tests/test_render_window.cpp.
 
+#include <string>
+
 namespace reasampler::capture {
 
 // Frames the [startSeconds, endSeconds) window occupies at `sampleRate`. Both
@@ -42,4 +45,28 @@ bool itemExtentPrintsWindow(double reqStart, double reqEnd,
                             double itemStart, double itemEnd,
                             int sampleRate);
 
+// --- Diagnostics: where a short render lost its frames ------------------------
+
+// The frames this window would hold if its END were resolved on a whole-millisecond
+// grid, floored, instead of exactly. Two live short renders (48 kHz, TailMode::None)
+// matched this count to the frame, which is the entire reason it exists.
+//
+// A COINCIDENCE OF COUNTS, not a claim about how anything resolved the end: nothing
+// renders from this number and no capture path asks for it. Whole-millisecond values
+// are recognized within a nanosecond, because a decimal millisecond is not always one
+// in binary (0.029 * 1000 lands just below 29) and a bare floor would drop a
+// millisecond from a window already on the grid. A nanosecond is far under one frame
+// at any rate we render, so a real sub-millisecond remainder still floors.
+long long msFlooredEndFrameCount(double startSeconds, double endSeconds,
+                                 int sampleRate);
+
+// The sentence a capture prints when the render bounds it handed REAPER did not read
+// back unchanged — the requested window, what came back, and both frame counts at
+// `sampleRate` (omitted when the rate is unknown). EMPTY when both edges read back
+// bit-identical, which is the only answer proving the request crossed into REAPER
+// intact; a caller prints this only when it is non-empty.
+std::string describeBoundsDrift(double reqStart, double reqEnd,
+                                double storedStart, double storedEnd,
+                                int sampleRate);
+
 }  // namespace reasampler::capture