fix: battle screens keep their composition when they open a menu or prompt

BATTLE SIZE "fixed" draws the battle as a discrete letterbox rather than
filling the window, and BATTLE BG "world" composes it over the live map.
Everything the battle then opens broke out of that composition, because
each piece of the frame's geometry was read off a fact about THIS FRAME
instead of about the battle:

* Renderer:uiScale follows the survey zoom only while a world is behind
  the UI, gated on worldActive -- this frame's world pass.  PartyMenu and
  ListMenu are opaque, so pushing one makes StateStack:visibleBase skip
  the map, no world pass runs, and the menu loses the step-down and blits
  a whole integer scale larger than the battle it just covered.  Held
  with uiWorldHold, the same whole-stack rule uiFill and the battle dim
  already use.  ("fill" hid this: it overrides the scale outright.)

* Game:draw started the frame at visibleBase, so that same opaque menu
  cut the overworld -- and the world pass with it -- out of the frame
  entirely, collapsing a "world" backdrop to endFrame's flat black clear.
  A world-bg battle now keeps the frame starting from underneath itself
  (drawBaseInStack).  Only the START of the draw moves; the clear stays
  keyed to the real visibleBase, so the menu still gets its opaque canvas
  and draws exactly as before.

* worldZones was keyed to that same clear base, so it came out nil for a
  frame whose world pass HAD run -- dropping endFrame's world blit onto
  the UI zone list instead, smearing the party menu's own HP-bar palettes
  across a world-canvas-sized image.  Keyed to whether the map drew.

* endFrame's letterbox clear read letterboxWhite off visibleBase alone,
  so an opaque menu over a BG "white" battle flipped its surround to
  black the same way.  Same whole-stack hold.

* ChoiceBox bottom-anchored unconditionally, docking it to the WINDOW
  edge.  That is only right when it rides the dialogue box beneath it,
  which is anchored there too; TextBox now passes the anchor and nothing
  else does, so the battle's switch offer and the shop/PC confirms stay
  over the screen that pushed them.

* TextBox anchors likewise: a battle is a self-contained SCREEN, not the
  window, and pokered prints its text box in the same 160x144 tilemap as
  the HUD.  The caught-mon nickname prompt was landing a whole letterbox
  below the blanked battle field it is printed on.  BattleState.holdsUI-
  Anchors holds setUIAnchor off while a battle is in the stack; the
  overworld's own dialogue box still docks to the screen edge.

src/core/Game.lua

@@ -310,6 +310,49 @@ function Game.wideBattleInStack(stack)
   return nil
 end
 
+-- Whether a state on the stack composes its own screen and so wants the
+-- edge anchors held off (BattleState.holdsUIAnchors).  Whole-stack, like
+-- everything else here: the text box and YES/NO a battle puts up are states
+-- of their own sitting above it, and they are exactly the elements that must
+-- stay inside the battle's composition rather than dock to the window.
+function Game.uiAnchorsHeldInStack(stack)
+  for i = #(stack and stack.states or {}), 1, -1 do
+    local state = stack.states[i]
+    if state and state.holdsUIAnchors then return true end
+  end
+  return false
+end
+
+-- Where Game:draw starts drawing this frame.  Normally the topmost opaque
+-- state (StateStack:visibleBase) -- but BATTLE BG "world" composes the battle
+-- over the LIVE map, and an opaque state pushed on top of it (the party menu,
+-- the bag) becomes that base, cutting the overworld -- and with it the world
+-- pass -- out of the frame entirely.  The backdrop the battle established
+-- then collapses to endFrame's flat black clear for as long as the menu is
+-- up.  So a world-bg battle keeps the frame starting from underneath itself
+-- until it leaves the stack, the same hold uiFill and the dim already use.
+--
+-- Only the START of the draw moves.  The clear stays keyed to the real
+-- visibleBase, so the menu still gets its opaque canvas and draws exactly as
+-- before; what changes is the window AROUND its letterbox, which keeps
+-- showing the map instead of going black.  Both menus fill their own
+-- 160x144 field first, so nothing beneath them shows through it.
+function Game.drawBaseInStack(stack, visibleBase)
+  local states = stack and stack.states or {}
+  for i = visibleBase - 1, 1, -1 do
+    local state = states[i]
+    if state and state.bgMode and state:bgMode() == "world" then
+      -- restart the search from under the battle: the highest opaque state at
+      -- or below it (the overworld), not the menu sitting over it
+      for j = i, 1, -1 do
+        if states[j].isOpaque then return j end
+      end
+      return 1
+    end
+  end
+  return visibleBase
+end
+
 -- Shift classic SGB zones to the centred UI. A full-width base zone extends
 -- into both margins, keeping the canvas' paper color continuous; narrower
 -- sprite and status zones move with the classic UI content.
@@ -335,6 +378,10 @@ function Game:draw()
   -- white clear
   local base = self.stack:visibleBase()
   local worldBelow = self.stack.states[base] == self.overworld
+  -- a world-bg battle keeps the map drawing under whatever it opened, so the
+  -- world pass can run for a frame whose CLEAR is still an opaque menu's
+  local drawFrom = Game.drawBaseInStack(self.stack, base)
+  local worldDrawn = self.stack.states[drawFrom] == self.overworld
   -- A wide battle holds its 304px surface through every menu or prompt it
   -- opens. States that do not draw the wide battle composition are centred
   -- in that surface below, so their classic coordinates and hit testing stay
@@ -359,8 +406,18 @@ function Game:draw()
   -- the stack for the same reason as uiFill above -- a prompt opened during
   -- the battle must not drop the dim for a frame.
   Renderer.battleDim = Game.worldBgBattleDim(self.stack)
+  -- ...and for the same reason the UI's own scale has to know the world is
+  -- still the backdrop while an opaque menu covers it.  Renderer:uiScale
+  -- steps the UI down with the survey zoom only while a world is behind it,
+  -- gated on this frame's world pass -- which the party menu and the bag end
+  -- by being opaque.  Without this hold they lose the step-down and blit at
+  -- full fit scale over a battle drawn at the zoomed-out one.
+  Renderer.uiWorldHold = Renderer.battleDim ~= nil
+  -- ...and a battle keeps its dialogue box and YES/NO inside its own screen
+  -- instead of letting them dock to the window edge.
+  Renderer.uiAnchorHold = Game.uiAnchorsHeldInStack(self.stack)
   Renderer:beginFrame(worldBelow)
-  for i = self.stack:visibleBase(), #self.stack.states do
+  for i = drawFrom, #self.stack.states do
     local state = self.stack.states[i]
     local wideState = state and state.isWideBattleLayout
       and state:isWideBattleLayout()
@@ -397,7 +454,13 @@ function Game:draw()
   if ModRuntime.wantsHook("render.zones") then
     zones = ModRuntime.call("render.zones", sameZones, self, zones)
   end
-  if worldBelow and self.overworld.sgbWorldZones then
+  -- Keyed to whether the map actually DREW, not to whether it is the clear's
+  -- base: an opaque menu over a world-bg battle still renders the world pass
+  -- (drawBaseInStack), and leaving worldZones nil there drops endFrame's
+  -- world blit onto the UI zone list instead -- the party menu's own HP-bar
+  -- palettes, in 160x144 space, smeared across a world-canvas-sized image.
+  -- That is the offset, red-for-green map behind the menu.
+  if worldDrawn and self.overworld.sgbWorldZones then
     worldZones = self.overworld:sgbWorldZones()
   end
   local viewport = Renderer:endFrame(zones, worldZones)