Fix eight review findings on the render-bounds diagnostics

Corrects a false comment example, fixes two tests that couldn't detect their
own regressions, adds two more read-back checkpoints around Main_OnCommand so
a drift report self-locates, guards a spurious zero-vs-zero coincidence match,
and softens two sentences that overclaimed cause or defect.

src/shell/capture/render_bounds_gate.cpp

@@ -100,12 +100,17 @@ BoundsVerdict checkRenderedBounds(const std::string& renderedPath,
     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.
+    // matched to the frame: the END alone floored to the millisecond. Checked against
+    // the END only -- a refusal whose START is also off-grid and independently floored
+    // would not match this shape, and this note's silence on that refusal is this
+    // check not covering it, not the coincidence breaking. Excludes 0, which every
+    // sub-millisecond window (a legitimate day-one capture) also floors to, and which
+    // would otherwise match a render that produced nothing. A count coincidence only —
+    // it does not establish how the render resolved anything.
+    const long long msFlooredEnd =
+        msFlooredEndFrameCount(request.startSeconds, request.endSeconds, rate);
     const std::string msNote =
-        actualFrames == msFlooredEndFrameCount(request.startSeconds,
-                                               request.endSeconds, rate)
+        (msFlooredEnd > 0 && actualFrames == msFlooredEnd)
             ? " 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();