render: add cross-platform desktop companion display

src/core/HostShell.lua

@@ -272,6 +272,38 @@ function HostShell.quote(s)
   return "'" .. s:gsub("'", "'\\''") .. "'"
 end
 
+-- Launch another instance of this packaged app without waiting for it.  The
+-- same path works on all process-capable desktop hosts; only the shell's
+-- background spelling differs.  Source checkouts include their game folder,
+-- while fused releases and AppImages already carry it in the executable.
+function HostShell.spawnSelfDetached(args)
+  if not require("src.core.Platform").canSpawnProcess() then return false end
+  local fs = love and love.filesystem
+  if not (fs and fs.getExecutablePath) then return false end
+  local executable = os.getenv("APPIMAGE") or fs.getExecutablePath()
+  if type(executable) ~= "string" or executable == "" then return false end
+
+  local argv = {}
+  local fused = fs.isFused and fs.isFused()
+  if not os.getenv("APPIMAGE") and not fused and fs.getSource then
+    argv[#argv + 1] = fs.getSource()
+  end
+  for _, value in ipairs(args or {}) do argv[#argv + 1] = tostring(value) end
+
+  local command = HostShell.quote(executable)
+  for _, value in ipairs(argv) do
+    command = command .. " " .. HostShell.quote(value)
+  end
+  local osName = love.system and love.system.getOS and love.system.getOS()
+  if osName == "Windows" then
+    command = 'start "" /b ' .. command .. " >NUL 2>&1"
+  else
+    command = HostShell.envPrefix() .. command .. " >/dev/null 2>&1 &"
+  end
+  local ok, _, code = os.execute(command)
+  return ok == true or ok == 0 or code == 0
+end
+
 -- MEMOISED per Lua state (so once per thread).  This used to spawn a whole
 -- `curl --version` process on every single fetch -- twice for a GET through
 -- the Android-bridge fallback -- which doubled the number of spawns the lock