render: add render.compose seam + second-screen bridge

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).

src/render/SecondScreen.lua

@@ -0,0 +1,59 @@
+-- Bridge to native secondary-display output (Android Presentation). The C
+-- functions live in mobile/android/love/src/jni/love/src/common/android.cpp.
+-- Everything is guarded: off Android, or if the symbols cannot be resolved,
+-- this stays inert and the renderer keeps the in-window stacked layout.
+
+local SecondScreen = {}
+local C = nil
+
+local function log(msg)
+  pcall(function() require("src.core.Logger").info("SecondScreen: %s", msg) end)
+end
+
+do
+  local ok, ffi = pcall(require, "ffi")
+  if not (ok and ffi) then
+    log("ffi unavailable (not LuaJIT); second display disabled")
+  else
+    pcall(ffi.cdef, [[
+      int love_android_secondary_ready();
+      void love_android_push_secondary(const void *rgba, int w, int h);
+      void love_android_secondary_enable(int on);
+    ]])
+    local okLib, lib = pcall(ffi.load, "love")
+    if okLib and lib and pcall(function() return lib.love_android_secondary_ready end) then
+      C = lib
+      log("bridge linked via ffi.load('love')")
+    elseif pcall(function() return ffi.C.love_android_secondary_ready end) then
+      C = ffi.C
+      log("bridge linked via default namespace")
+    else
+      log(("bridge symbols not found (ffi.load ok=%s); second display disabled")
+        :format(tostring(okLib)))
+    end
+  end
+end
+
+function SecondScreen.usable()
+  return C ~= nil
+end
+
+function SecondScreen.available()
+  if not C then return false end
+  local ok, r = pcall(C.love_android_secondary_ready)
+  return ok and r ~= 0
+end
+
+function SecondScreen.push(imageData, w, h)
+  if not C or not imageData then return false end
+  return pcall(function()
+    C.love_android_push_secondary(imageData:getFFIPointer(), w, h)
+  end)
+end
+
+function SecondScreen.setEnabled(on)
+  if not C then return end
+  pcall(function() C.love_android_secondary_enable(on and 1 or 0) end)
+end
+
+return SecondScreen