capture: name the render source in the exact-bounds refusal, and put its one-frame tolerance under test

The tolerance is unchanged and now derived, not assumed: frameCountFor lands in {floor(L), ceil(L)}, so a non-frame-aligned window can never miss by more than a frame. Naming the source is what tells a self-bounding render from a short one.

tests/test_render_settings.cpp

@@ -10,6 +10,7 @@
 
 #include <cmath>
 #include <cstdio>
+#include <cstring>
 #include <set>
 #include <string>
 
@@ -63,6 +64,28 @@ static void testRealtimeIsUnsupportedOffline() {
     CHECK(!renderSettingsFor(SourceMode::Realtime, 1.0).supported);
 }
 
+static void testEveryRenderSourceIsNameable() {
+    // The bounds refusal quotes this label to say WHICH render source missed the
+    // window, so every mode must name itself and no two may read alike.
+    const SourceMode all[] = {
+        SourceMode::MasterMix, SourceMode::TimeSelection, SourceMode::SelectedTracks,
+        SourceMode::SelectedItems, SourceMode::RazorArea, SourceMode::Realtime,
+    };
+    for (std::size_t i = 0; i < sizeof(all) / sizeof(all[0]); ++i) {
+        const char* label = renderSourceLabel(all[i]);
+        CHECK(label != nullptr && label[0] != '\0');
+        CHECK(std::strcmp(label, "unknown") != 0);
+        for (std::size_t j = i + 1; j < sizeof(all) / sizeof(all[0]); ++j)
+            CHECK(std::strcmp(label, renderSourceLabel(all[j])) != 0);
+    }
+    // The two the refusal must tell apart: a source that derives its own bounds vs
+    // the time-bounded render.
+    CHECK(std::strcmp(renderSourceLabel(SourceMode::SelectedItems),
+                      "selected media items") == 0);
+    CHECK(std::strcmp(renderSourceLabel(SourceMode::SelectedTracks),
+                      "selected tracks via master") == 0);
+}
+
 // --- tail: TailMode -> RENDER_* mapping (docs/product/capture-tail.md) --------
 
 static void testTailNoneIsExactBounds() {
@@ -395,6 +418,7 @@ int main() {
     testSelectedItemsSingleFile();
     testRazorSingleFile();
     testRealtimeIsUnsupportedOffline();
+    testEveryRenderSourceIsNameable();
     testTailNoneIsExactBounds();
     testTailAutoIsSurgicalTrim();
     testAutoTrimRatioDerivesFromDb();