build(streamer): make ZED SDK optional

README.md

@@ -45,6 +45,17 @@ cmake -B build -S .
 cmake --build build
 ```
 
+When the ZED SDK is available, the build also enables `zed_svo_to_mcap` and
+`zed_svo_to_mp4` automatically. When the SDK is absent, those tools are skipped
+and the main streamer plus non-ZED testers still build normally.
+
+`zed_svo_grid_to_mp4` remains optional and additionally requires OpenCV. Disable
+it explicitly with:
+
+```bash
+cmake -B build -S . -DCVMMAP_BUILD_ZED_SVO_GRID_TO_MP4=OFF
+```
+
 ```bash
 # Use a local cv-mmap build tree
 cmake -B build -S . \
@@ -60,6 +71,8 @@ ls -la build/{cvmmap_streamer,rtp_receiver_tester,rtmp_stub_tester}
 
 ### ZED SVO/SVO2 To MP4
 
+This tool is only built when the ZED SDK is detected during CMake configure.
+
 The repo also includes an offline conversion tool for the left ZED color stream:
 
 ```bash
@@ -130,6 +143,9 @@ The batch tool mirrors the common encoder options from `zed_svo_to_mp4`, skips e
 
 ### ZED SVO Grid To MP4
 
+This tool is only built when the ZED SDK is detected and
+`CVMMAP_BUILD_ZED_SVO_GRID_TO_MP4=ON`.
+
 Use the grid converter to merge four synced ZED recordings into a 2x2 CCTV-style MP4 with a Unix timestamp overlay in the top-left corner:
 
 ```bash
@@ -213,6 +229,9 @@ timestamp,activity,group_path,segment_dir,camera,relative_path
 
 ### Batch ZED Segments To MCAP
 
+This workflow depends on the `zed_svo_to_mcap` binary, which is only built when
+the ZED SDK is detected during CMake configure.
+
 Use the wrapper to recurse through a dataset root, run `zed_svo_to_mcap --segment-dir` on every matched multi-camera segment, and show one aggregate tqdm progress bar:
 
 ```bash