capture: close batch-quarantine silence, 0-byte asymmetry, and round-two doc overclaims

Batch captures now name the retained-render folder once instead of nothing; Auto/Manual tail modes refuse a 0-byte render like None does; VERIFICATION.md steps 1-3 no longer invite a false conclusion; docs/comments no longer overclaim.

src/core/capture/render_window.cpp

@@ -8,8 +8,9 @@ namespace reasampler::capture {
 
 namespace {
 
-// Round-to-nearest, so a position that sits mid-frame maps to the frame a render
-// of it prints rather than to the frame below it.
+// Round-to-nearest: the convention THIS module measures a window by, so a mid-frame
+// position maps to the closer frame boundary rather than always down. Not a claim
+// about how any renderer resolves that position -- see the header's caveat.
 long long frameIndexAt(double seconds, int sampleRate) {
     return std::llround(seconds * static_cast<double>(sampleRate));
 }