Fix render-bounds EXACT verdict: enumerate floored models instead of trusting grid membership

Grid-ness of an edge was a proxy for "no floor could explain this count," not
the test itself — equal remainders on both edges cancel under a full floor. Now
checks all three floored models directly and corrects the SHORT/LONG
floor-signature docs.

src/core/capture/render_window.h

@@ -85,20 +85,32 @@ bool isOnMillisecondGrid(double seconds);
 // at all. Always non-empty — a capture that answered nothing has to say so, or its
 // silence reads as a pass.
 //
-// EXACT never stands alone as proof: an END that sits on the millisecond grid prints
-// the SAME EXACT count whether the channel honored the window or floored it and landed
-// back on the grid by coincidence, so that case is called out in the sentence rather
-// than left to read as settled — same principle as the existing START-edge caveat.
+// EXACT never stands alone as proof: the observed count is checked against every
+// millisecond-floored model of the same window (start floored alone, end floored alone,
+// both together), and any model that reproduces it is named in the sentence. Grid
+// membership on an edge is a PROXY for that collision, not the test itself — remainders
+// on the two edges can cancel under a full floor even when NEITHER edge sits on the
+// grid, and a remainder under half a frame collides with a floored edge without ever
+// registering as off-grid at all. Checking the models directly is what a grid test on
+// either edge alone cannot do.
 //
 // A non-zero delta that still falls inside the gate's own tolerance (renderHonoredBounds)
 // is tagged "(WITHIN TOLERANCE)" — that is the gate's ordinary edge-convention slack, not
-// evidence of the millisecond floor; a bare SHORT/LONG, or a delta matching
-// msFlooredEndFrameCount exactly, is the floor's signature.
+// evidence of the millisecond floor. The floor's signature is ONLY the "floored to the
+// millisecond" sentence (a delta matching msFlooredEndFrameCount exactly); a bare
+// SHORT with no such sentence means the shortfall's cause is unestablished, and LONG can
+// never be the floor's signature — a floor only removes frames, never adds them. A
+// window whose start and end sit in the same millisecond bucket makes msFlooredEndFrameCount
+// equal the exact count, so a real one-frame floor there reads as a bare SHORT (WITHIN
+// TOLERANCE) with no floor sentence at all — that combination is not evidence the floor
+// didn't happen, just a case this diagnostic can't see into.
 //
 // `sampleRate <= 0` means the landed file was never measured: a tail mode adds frames by
 // design and is not judged, an empty render has none, and a render whose layout failed to
 // parse or declared no sample rate is refused before it can be judged either — the
-// sentence then says the run answered nothing rather than inventing a comparison.
+// sentence then says the run answered nothing rather than inventing a comparison. A
+// window that rounds to 0 frames at this rate reads NOT JUDGED the same way: a 0-frame
+// render against a 0-frame window is not a comparison either.
 //
 // `bypassingSourceLabel`, when non-null and non-empty, means the render source itself
 // defined the window (render_settings::sourceBypassesBoundsChannel) — `channelLabel` was