review: address Copilot feedback on ProRes encoder integration

video.h:
  is_known_video_format() now uses `< SUNSHINE_FORMAT_COUNT` instead of
  the hardcoded `<= SUNSHINE_FORMAT_PRORES` upper bound. Future codecs
  are now a one-enum-bump change.

video.cpp:
  require_prores binds to the immutable config::video.prores_mode rather
  than the mutable active_prores_mode. adjust_encoder_constraints() may
  demote active_prores_mode to 0; reading the immutable source keeps the
  "user explicitly asked for forced ProRes" intent intact so we fail
  loudly instead of silently picking a non-ProRes encoder.

cmake/dependencies/macos.cmake:
  Pull in the SCREEN_CAPTURE_KIT_LIBRARY REQUIRED change too — this PR
  stacks on the SCK backend PR and needs the same SDK enforcement.

Addresses Copilot inline feedback from the closed upstream PR cycle.

Author: Nottlespike

src/video.cpp

@@ -2968,7 +2968,14 @@ namespace video {
     active_hevc_mode = config::video.hevc_mode;
     active_av1_mode = config::video.av1_mode;
     active_prores_mode = config::video.prores_mode;
-    const bool require_prores = active_prores_mode >= 2;
+    // Bind `require_prores` to the user-configured value, NOT to the
+    // mutable `active_prores_mode` global. `adjust_encoder_constraints`
+    // below may demote `active_prores_mode` to 0 when no encoder supports
+    // ProRes; reading from the immutable config source keeps the
+    // "user explicitly asked for forced ProRes" intent intact across that
+    // demotion, so we can fail loudly instead of silently picking a
+    // non-ProRes encoder.
+    const bool require_prores = config::video.prores_mode >= 2;
     last_encoder_probe_supported_ref_frames_invalidation = false;
 
     auto adjust_encoder_constraints_hevc = [&](encoder_t *encoder) {

src/video.h

@@ -31,7 +31,10 @@ namespace video {
   static_assert(SUNSHINE_FORMAT_PRORES == 3);
 
   inline bool is_known_video_format(int video_format) {
-    return video_format >= SUNSHINE_FORMAT_H264 && video_format <= SUNSHINE_FORMAT_PRORES;
+    // Express the upper bound via SUNSHINE_FORMAT_COUNT so adding a future
+    // codec is purely a matter of bumping the enum — no need to remember
+    // to update this predicate.
+    return video_format >= SUNSHINE_FORMAT_H264 && video_format < SUNSHINE_FORMAT_COUNT;
   }
 
   inline bool is_video_format_enabled_by_prores_gate(int video_format, int prores_mode) {