Premium highlight overlay styling (P3.3)

Replace the plain hard-cut white caption with a refined 48px caption, a soft drop
shadow, a thin subtle border, and a smooth alpha fade in and out (0.4s ramps)
within each overlay timeline window. Highlight renderer only; the multi-clip
renderer and overlay placement/safe-area logic are unchanged. Test added;
mvn -o verify green (246 tests, 0 failures).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bg76sLc43Wc3j5ZcLkboYR

src/main/java/org/example/videoclips/editing/HighlightFfmpegRenderer.java

@@ -555,9 +555,17 @@ public class HighlightFfmpegRenderer {
     }
 
     private String drawTextFilter(TextOverlay overlay) {
-        return "drawtext=text='%s':x=%s:y=%s:fontsize=54:fontcolor=white:borderw=2:bordercolor=black@0.75:enable='between(t\\,%s\\,%s)'"
+        String start = Double.toString(overlay.timelineStartSeconds());
-                .formatted(escapeDrawText(overlay.text()), overlayX(overlay.placement()), overlayY(overlay.placement()),
+        String end = Double.toString(overlay.timelineEndSeconds());
-                        overlay.timelineStartSeconds(), overlay.timelineEndSeconds());
+        // Smooth alpha fade: ramp up over the first 0.4 s, hold, ramp down over the last 0.4 s.
+        String alpha = "if(lt(t\\," + start + "+0.4)\\,(t-" + start + ")/0.4\\,"
+                + "if(lt(t\\," + end + "-0.4)\\,1\\,(" + end + "-t)/0.4))";
+        // Premium caption: refined size, soft drop shadow, thin subtle border for legibility.
+        return ("drawtext=text='%s':x=%s:y=%s:fontsize=48:fontcolor=white"
+                + ":shadowcolor=black@0.55:shadowx=2:shadowy=2:borderw=1:bordercolor=black@0.35"
+                + ":alpha='%s':enable='between(t\\,%s\\,%s)'")
+                .formatted(escapeDrawText(overlay.text()), overlayX(overlay.placement()),
+                        overlayY(overlay.placement()), alpha, start, end);
     }
 
     private String overlayX(String placement) {

src/test/java/org/example/videoclips/editing/HighlightFfmpegRendererTest.java

@@ -76,6 +76,20 @@ class HighlightFfmpegRendererTest {
                 .contains("[voice_line_0][voice_line_1]amix=inputs=2:duration=longest"));
     }
 
+    @Test
+    void stylesOverlaysWithSoftShadowAndAlphaFade() {
+        HighlightFfmpegRenderer renderer = renderer();
+        String filter = renderer.overlayFilter(List.of(
+                new TextOverlay("First light.", 0.5, 3.0, "center_safe", "fade", "hook")));
+
+        assertThat(filter)
+                .contains("drawtext=text='First light.'")
+                .contains("fontsize=48")
+                .contains("shadowcolor=black@0.55:shadowx=2:shadowy=2")
+                .contains("alpha='if(lt(t\\,0.5+0.4)")
+                .contains("enable='between(t\\,0.5\\,3.0)'");
+    }
+
     @Test
     void buildsMeasuredQaProbeCommands() {
         HighlightFfmpegRenderer renderer = renderer();