refactor(GUI): OnTop Boolean-assert contract (Kai-Entscheid)

Card 2e40bf — Kai's pin: crash loudly + caller sweep.

• 16 functions asserted: GuiButton, WideGuiButton/2/3, OutpostButton, ArrowButton, TrashButton, EditButton, LockButton, WideAutoFitButton, ItemButton, CrushButton, SaveNameButton, SaveNameErrorButton, UpgradePlayerButton, UpgradePyramidButton — AssertContract OnTop Boolean at the draw-fn boundary; nil-defaults removed
• ~50 call sites made explicit: enclosing OnTop where in scope; literal true for always-on-top contexts (modals, game-prep, options screens); DrawFilterChoice's 3 helper buttons got OnTop threaded rather than frozen
• DrawArrowButton Color slot nil-padded where needed
• Merge-order: QuantumPrediction:337 passes true here (rebase to OnTop if #1334 lands first); overlaps #1335/#1336 signature regions