Add gr.Examples block (gated on demo file presence)

The block renders only if examples/demo_codec_heatmap.mp4 exists in
the Space root, so checkouts without the binary still launch cleanly.
Three preset rows ready for the demo clip:
1. Selection · gradient · global (baseline)
2. Heatmap · combined · log · dynamic (readiness story)
3. Side-by-side · combined · GOP=8 · 2048 budget (full feature)

The video itself is uploaded separately via xet (binary files are not
allowed through plain git push to HF Spaces).

app.py

@@ -1122,6 +1122,46 @@ with gr.Blocks(**_BLOCK_KW) as demo:
                                 label="", language="json", lines=18,
                             )
 
+    DEMO_VIDEO = os.path.join(
+        os.path.dirname(os.path.abspath(__file__)),
+        "examples", "demo_codec_heatmap.mp4",
+    )
+    if os.path.exists(DEMO_VIDEO):
+        with gr.Group(elem_classes="ovc-card"):
+            gr.Markdown("### Demo video")
+            gr.Markdown(
+                "<small>Click any row below to load the bundled clip with a "
+                "preset configuration. Or upload your own video at the top.</small>"
+            )
+            gr.Examples(
+                examples=[
+                    [
+                        DEMO_VIDEO, 16, 14, 1024, 150000,
+                        "selection", 0.55, 0.0, 0.0,
+                        "gradient", False, 99.0, 0.55, "global",
+                    ],
+                    [
+                        DEMO_VIDEO, 16, 14, 1024, 150000,
+                        "heatmap", 0.55, 0.0, 0.0,
+                        "combined", True, 96.0, 0.55, "dynamic",
+                    ],
+                    [
+                        DEMO_VIDEO, 24, 14, 2048, 150000,
+                        "sbs", 0.55, 0.0, 0.0,
+                        "combined", True, 95.0, 0.55, "8",
+                    ],
+                ],
+                inputs=[
+                    video_in, sample_frames, patch_size, top_k, max_pixels,
+                    viz_mode, heatmap_alpha, start_sec, end_sec,
+                    saliency_signal, score_log_scale, bitcost_pct,
+                    fade_strength, gop,
+                ],
+                label="",
+                cache_examples=False,
+                examples_per_page=5,
+            )
+
     gr.HTML(
         '<div id="ovc-footer">'
         '<b>OneVision Encoder</b> · codec-style patch saliency demo · '