capture: refuse the multi-track ranged item render, silence the track's children and receives for it, and gate every exact-bounds capture on its frame count

src/core/capture/render_window.h

@@ -11,15 +11,21 @@ namespace reasampler::capture {
 // is a difference of frame indices rather than a rounded duration — two windows
 // of equal length at different offsets can legitimately differ by one frame.
 // Returns 0 for a non-positive rate or an empty/inverted window.
+//
+// The offline backend compares this against the rendered file's own frame count, so
+// exact-bounds failures surface as a refused capture rather than a wrong file. That
+// REAPER resolves the two edges the same way is UNVERIFIED — a DAW pass decides
+// whether the equality is exact or off by a frame.
 long long frameCountFor(double startSeconds, double endSeconds, int sampleRate);
 
 // 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
-// source needs no correction. Compared at frame resolution, because a sub-frame
-// difference prints the same frames. An unknown rate (<= 0) falls back to exact
-// equality, which can only send a window to the time-bounded render, never widen
-// one.
+// source is believed to need no correction (the bounds-override inference behind
+// that is unverified; src/core/capture/CLAUDE.md §Gotchas states what it rests on).
+// Compared at frame resolution, because a sub-frame difference prints the same
+// frames. An unknown rate (<= 0) falls back to exact equality, which can only send
+// a window to the time-bounded render, never widen one.
 bool itemExtentPrintsWindow(double reqStart, double reqEnd,
                             double itemStart, double itemEnd,
                             int sampleRate);