Expose a generic seam so a mod can drive a second screen without the
engine owning any dual-screen layout policy:
- render.compose hook in Renderer:endFrame hands a mod the finished
world + UI canvases, their SGB zones, the frame metrics,
Renderer:blitCanvas (lifted from the internal blit closure) and the
SecondScreen bridge. Return true to take over the window; no wrap (or
calling next) runs the normal single-window composite byte-for-byte.
- SecondScreen.lua + the Android Presentation bridge (love_android_
secondary_* in common/android.cpp, GameActivity secondary display)
as the optional physical-second-display transport.
No battle-render changes: a mod lays out the two screens (including any
battle split) itself. Ships with a unit test, no-mod parity via
gate_hooks, and docs/modding.md (D14).
Bart in 't Veld