Merge buttonPlacement and featureMenu into a single ButtonController module

[VampireChicken12]

Summary

Eliminate the bidirectional coupling between src/features/buttonPlacement/ and src/features/featureMenu/ by merging them into a single ButtonController class that owns all button DOM, menu DOM, fullscreen handling, theater observer, and shared tracking state.

This is Candidate 1 from the architecture exploration doc, next after the config type system (Closes #1348).

Current Architecture

FeatureButtonManager (registry)
  -> btn.add(config) -> addFeatureButton(8 params) -> enableFeatureMenu()
                                                     -> makeFeatureButton()
                                                     -> placeButton()
                                                     -> addFeatureItemToMenu()  (for feature_menu)
                                                     -> trackButton()
                                                       -> startFullscreenObserver()
                                                         -> handleFullscreenChange()
  -> btn.remove()   -> removeFeatureButton()

Friction Points

1. Duplicate container creation

#yte-right-controls-container created with identical code in two files:

• buttonPlacement/utils.ts (placeButton, player_controls_right case)
• featureMenu/index.ts (createFeatureMenuButton)

If selectors drift apart, one placement breaks silently.

2. Three parallel tracking systems

• trackedButtons (Map, buttonPlacement/utils.ts) - button state for fullscreen transitions
• featuresInMenu (Set, featureMenu/utils.ts) - menu item presence
• buttonState (Map, featureButtonManager.ts) - enabled/placement per feature

Fullscreen transitions must manually keep all three in sync.

3. 8-positional-parameter surface

addFeatureButton() takes 8 positional params. Every feature's btn.add() repeats the same boilerplate.

4. Generic unsoundness

addFeatureItemToMenu accepts ListenerType<Toggle> but gets ListenerType<boolean> from handleFullscreenChange.

5. Error recovery gap in addFeatureButton

Early return in placement logic runs before trackButton() - if button creation rejects, tracking state goes inconsistent.

6. Theater observer in wrong module

Theater observer lives in buttonPlacement/utils.ts but manages the "below player" container - a cross-cutting DOM concern.

Files Involved

• src/features/buttonPlacement/index.ts - addFeatureButton, removeFeatureButton, getPlacementSelector
• src/features/buttonPlacement/utils.ts - makeFeatureButton, placeButton, trackButton, handleFullscreenChange, theater observer
• src/features/buttonPlacement/fullscreen.ts - fullscreen observer
• src/features/featureMenu/index.ts - enableFeatureMenu, createFeatureMenuButton, setupFeatureMenuEventListeners
• src/features/featureMenu/utils.ts - addFeatureItemToMenu, removeFeatureItemFromMenu, featuresInMenu
• src/features/_registry/featureButtonManager.ts - orchestrator

Proposed Boundary

Create ButtonController that owns:

• DOM container creation (#yte-right-controls-container, #yte-button-container)
• Button DOM creation (makeFeatureButton)
• Menu DOM creation (addFeatureItemToMenu / removeFeatureItemFromMenu)
• Fullscreen observer + handler
• Theater observer + handler
• Combined tracking state (replacing trackedButtons + featuresInMenu)

The outward API surface shrinks to roughly:

class ButtonController {
  async addButton(name, config, icon, listener, isToggle)
  async removeButton(name, placement?)
  getButton(name): HTMLElement | null
}

featureMenu/utils.ts gets inlined into the controller. featureMenu/index.ts keeps only createFeatureMenuButton / setupFeatureMenuEventListeners (the menu toggle UI).

Steps

1. Create ButtonController class with combined state + all methods
2. Inline fullscreen.ts observer into the controller
3. Inline featureMenu/utils.ts exports into the controller
4. Deduplicate container creation
5. Collapse addFeatureButton 8 params into options object
6. Update all 12 feature index.ts files to call buttonController.addButton
7. Remove old files: buttonPlacement/index.ts, buttonPlacement/utils.ts, buttonPlacement/fullscreen.ts, featureMenu/utils.ts
8. Prune re-exports and dead code