feat: UI LAYOUT option, centered by default

Edge docking and zoom-linked UI scaling shipped as unconditional
behaviour. Both are departures from how the port composed the screen, so
they become a setting instead: UI LAYOUT = CENTERED (the default) or
DYNAMIC.

CENTERED is a fixed letterbox. Elements stay where they were drawn in the
160x144 canvas, and the UI does not follow the survey zoom, so screen
furniture neither moves nor resizes under the player. That is what the
pre-anchoring builds did. DYNAMIC is the current behaviour, unchanged.

Both halves matter together: gating only the anchoring would stop the
dialogue box moving but leave it resizing with the zoom, which is the same
complaint in a different form.

Gated at Renderer:setUIAnchor and Renderer:uiScale rather than at each
caller, so one switch covers the dialogue box, its YES/NO, the START menu
and anything anchored later, and no caller knows the option exists.
Game.dynamicUI answers true only for an explicit "dynamic", so a save
written before this keeps the layout it already had.

Independent of it, deliberately: BATTLE SIZE still works under either mode
(uiFill overrides the scale later, in endFrame), and a battle still holds
its own prompts inside its screen under DYNAMIC.

Also includes the Oak intro fix (previously #674): the speech fills white
over the UI canvas while its dialogue box docks to the window edge, so
under DYNAMIC black showed between the two. letterboxWhite closes it, and
the shrink beat's replica box rides the same anchor as the real box it
stands in for.

src/ui/OptionsMenu.lua

@@ -191,6 +191,23 @@ local function buildRows(game)
         o.battleBg = order[(cur - 1 + (dir or 1)) % #order + 1]
         return true
       end },
+    -- CENTERED is a fixed letterbox: elements stay inside the 160x144 canvas
+    -- and the UI does not follow the survey zoom, so nothing moves or resizes
+    -- under the player.  The composition the port shipped with.  DYNAMIC docks
+    -- the dialogue box to the window's bottom edge and the START menu to its
+    -- top right, and steps the UI down with the zoom -- easier to read zoomed
+    -- out, but it moves furniture the original never moved, so it is opt-in.
+    -- BATTLE SIZE is independent of this and works under either.
+    { id = "uiLayout", label = Strings("UI LAYOUT"),
+      value = function(g)
+        return g.save.options.uiLayout == "dynamic" and Strings("DYNAMIC")
+               or Strings("CENTERED")
+      end,
+      step = function(g)
+        local o = g.save.options
+        o.uiLayout = o.uiLayout == "dynamic" and "centered" or "dynamic"
+        return true
+      end },
     { id = "ruleset", label = Strings("RULESET"),
       value = function(g) return rulesetName(g) end,
       step = function(g, dir)