feat(host): add optional display lifecycle backend

src/core/HostDisplay.lua

@@ -0,0 +1,41 @@
+-- Optional native-host display lifecycle.
+--
+-- The engine always owns simulation and drawing. A packaged host may install
+-- one backend to observe per-frame updates and prepare/finalize a render target
+-- around an otherwise unchanged draw. With no backend installed every method
+-- is a no-op, which is the normal desktop and mobile path.
+--
+-- Backend methods are optional:
+--   backend:update(dt)
+--   backend:beginFrame(kind, subject)
+--   backend:endFrame(kind, subject)
+--
+-- `kind` is "editor", "touch_editor", "launcher", or "game". `subject` is
+-- the object whose existing draw method runs between beginFrame and endFrame.
+local HostDisplay = {}
+
+local backend
+
+function HostDisplay.setBackend(value)
+  if value ~= nil and type(value) ~= "table" then
+    error("host display backend must be a table or nil", 2)
+  end
+  backend = value
+end
+
+function HostDisplay.update(dt)
+  local fn = backend and backend.update
+  if fn then return fn(backend, dt) end
+end
+
+function HostDisplay.beginFrame(kind, subject)
+  local fn = backend and backend.beginFrame
+  if fn then return fn(backend, kind, subject) end
+end
+
+function HostDisplay.endFrame(kind, subject)
+  local fn = backend and backend.endFrame
+  if fn then return fn(backend, kind, subject) end
+end
+
+return HostDisplay