feat: add mcap recorder control and cnats providers

Register an MCAP recorder service on the streamer control subjects, reuse the shared recording request and status model, and expose the zed recording preview/conversion helper.

This also replaces the temporary cnats boolean with the explicit CVMMAP_CNATS_PROVIDER modes and documents the supported system and workspace build paths.

README.md

@@ -35,11 +35,24 @@ sudo pacman -S cmake gstreamer gst-plugins-base gst-plugins-good \
 
 ### Build
 
+`cvmmap-streamer` uses `CVMMAP_CNATS_PROVIDER` to decide how `cnats` is resolved:
+
+- `system` (default): use an installed `cnats` package, typically from a top-level `cv-mmap` install under a standard prefix like `/usr/local`
+- `workspace`: use the local `cv-mmap` build-tree exports
+
 ```bash
 cmake -B build -S .
 cmake --build build
 ```
 
+```bash
+# Use a local cv-mmap build tree
+cmake -B build -S . \
+  -DCVMMAP_CNATS_PROVIDER=workspace \
+  -DCVMMAP_LOCAL_ROOT=/path/to/cv-mmap
+cmake --build build
+```
+
 **Verify binaries exist:**
 ```bash
 ls -la build/{cvmmap_streamer,rtp_receiver_tester,rtmp_stub_tester}