fix(macos): bundle SDL3 for sdl2-compat at install time

Homebrew replaced its `sdl2` formula with `sdl2-compat`, a compatibility
layer that provides the SDL2 API by `dlopen`-ing the real SDL3 library
at runtime. The macOS CI build (`build-macos.yml`) pulls this via `brew
install mpv`, so the app now links `libSDL2-2.0.0.dylib` which is
actually `sdl2-compat`, and every nightly ships it in
`Contents/Frameworks`.

SDL3 is not a linker dependency, so it never shows up in `otool -L`
output and the `CompleteBundleMac` fixup loop never copies it into the
app bundle. At runtime `sdl2-compat` looks for `libSDL3.dylib` relative
to its own loader path, cannot find it, prints "Failed loading SDL3
library." and aborts during dyld initialization, since the library is
missing. This regression appeared once Homebrew's SDL2 became the
`sdl2-compat` shim, so older nightly builds (built against genuine SDL2)
are unaffected.

This detects `sdl2-compat` inside the bundle (it is the only
`libSDL2-2.0.0.dylib` containing the `libSDL3.dylib` loader string) and
explicitly copies libSDL3 from the Homebrew prefix into
`Contents/Frameworks/libSDL3.dylib`, using the same `install-id`
convention as the other bundled dylibs so `codesign` and the existing
fixup steps keep working. If SDL3 cannot be found at install time a
warning is printed instead of failing silently.

Fixes #1225

CMakeModules/CompleteBundleMac.cmake.in

@@ -230,6 +230,45 @@ endwhile()
 
 message(STATUS "Library path fixing complete")
 
+#
+# sdl2-compat: Homebrew's sdl2 formula is now a compatibility layer that loads
+# the real SDL3 library at runtime via dlopen. Since SDL3 is not a linker
+# dependency, it is invisible to otool and is never picked up by the fixup loop
+# above. Detect sdl2-compat and bundle libSDL3 explicitly.
+#
+set(sdl2_compat_lib "${framework_path}/libSDL2-2.0.0.dylib")
+if(EXISTS "${sdl2_compat_lib}")
+  execute_process(
+    COMMAND strings -a "${sdl2_compat_lib}"
+    OUTPUT_VARIABLE sdl2_compat_strings
+    ERROR_QUIET
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+  if(sdl2_compat_strings MATCHES "libSDL3\\.dylib")
+    set(sdl3_target "${framework_path}/libSDL3.dylib")
+    if(NOT EXISTS "${sdl3_target}")
+      set(sdl3_source "")
+      foreach(candidate "${brew_lib}/libSDL3.dylib" "${brew_lib}/libSDL3.0.dylib")
+        if(EXISTS "${candidate}")
+          get_filename_component(sdl3_source "${candidate}" REALPATH)
+          break()
+        endif()
+      endforeach()
+      if(sdl3_source)
+        message(STATUS "sdl2-compat detected, bundling SDL3 into ${framework_path}")
+        file(COPY "${sdl3_source}" DESTINATION "${framework_path}")
+        get_filename_component(sdl3_source_name "${sdl3_source}" NAME)
+        if(NOT "${sdl3_source_name}" STREQUAL "libSDL3.dylib")
+          file(RENAME "${framework_path}/${sdl3_source_name}" "${sdl3_target}")
+        endif()
+        execute_process(COMMAND install_name_tool -id "@executable_path/../Frameworks/libSDL3.dylib" "${sdl3_target}" ERROR_QUIET)
+      else()
+        message(WARNING "sdl2-compat detected but libSDL3 was not found in ${brew_lib}; the app will fail at runtime with 'Failed loading SDL3 library.'")
+      endif()
+    endif()
+  endif()
+endif()
+
 # List all bundled libraries for verification
 file(GLOB final_dylibs "${framework_path}/*.dylib")
 message(STATUS "=== Bundled libraries in Frameworks ===")