refactor: DeckLinkReceiver readability and error handling

DeckLinkReceiver:
- Group private members with section comments (SDK objects / sync / frame state)
- Change wait_for_format timeout param from uint64_t to int (matches
  std::chrono::milliseconds and all call sites)

decklinkin main.cpp:
- Wrap start_capture in try/catch — logs error and returns cleanly on
  device init failure (consistent with NDIInNode pattern)
- Remove redundant zero-init on frame_buf (vector<uint8_t> zero-inits anyway)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/DeckLinkReceiver.hpp

@@ -68,7 +68,7 @@ public:
         if (r != S_OK) throw std::runtime_error("Could not start streams");
     }
 
-    bool wait_for_format(uint64_t timeout_ms = 5000) {
+    bool wait_for_format(int timeout_ms = 5000) {
         std::unique_lock<std::mutex> lk(mutex);
         return format_cv.wait_for(lk, std::chrono::milliseconds(timeout_ms),
             [this] { return format_detected; });
@@ -173,16 +173,19 @@ private:
         std::atomic<int32_t> ref_count{1};
     };
 
+    // DeckLink SDK objects
     std::vector<IDeckLink*>  raw_devices;
     IDeckLink*               selected_device = nullptr;
     IDeckLinkInput*          decklink_input  = nullptr;
     InputCallback*           input_callback  = nullptr;
 
+    // Synchronisation — one mutex guards all shared state below
     std::mutex              mutex;
-    std::condition_variable format_cv;
-    std::condition_variable frame_cv;
+    std::condition_variable format_cv;   // signalled when format is detected
+    std::condition_variable frame_cv;    // signalled when a frame arrives
     bool                    format_detected = false;
 
+    // Frame state — written by InputCallback, read by wait_for_frame (both under mutex)
     std::vector<uint8_t>    frame_buffer;
     uint32_t                frame_row_bytes = 0;
     int                     frame_width     = 0;