docs: add Controls/index.md — the missing per-directory map for Phase-2 UserControls

Phase 2 (refactor-commit-12..18) created Controls/ with 6 UserControls that
split MainWindow.xaml, but no index.md existed — a gap against the schema.md
rule (one index.md per code directory). Documents each control, the window↔
control contract, and the Phase-2 landmines (window-resources converter rule,
control xmlns, name-looked-up host stubs, dropped-Border).

Controls/index.md

@@ -0,0 +1,58 @@
+# Controls — index
+
+The phase-2 carved pieces of what used to be one monolithic `MainWindow.xaml`
+(1,681 lines → a 123-line shell). Each is a `UserControl` in the `ytLive.Controls`
+namespace, extracted one roll-back commit at a time (refactor-commit-12..18).
+See [`schema.md`](../schema.md) for the memory-map conventions.
+
+| File | Purpose |
+|------|---------|
+| `BottomBar.xaml(.cs)` | Footer (Row 3): gear menu (stop/mic/game-mute/quality dropdown), stream stats (bitrate/fps/dropped/duration/health), resolution dropdown |
+| `HealthBanner.xaml(.cs)` | Row 1 status banner + the 7 value→visibility converters live in the app theme, not here |
+| `TopBar.xaml(.cs)` | Row 0 stream controls: REC/ON-AIR pills+signs, elapsed timer, start/stop, account avatar/name (`RefreshAvatar()`) |
+| `PreviewPane.xaml(.cs)` | Row 1 Col 1 center: the 1920×1080 `OverlayCanvas`/`SelectionOverlay`/`BrandFlashLayer`/`SocialBarElement`, text pull-out drawer, and the live-controls row (TRAX / game / mic meters + sliders + speakers + socials) |
+| `SceneThumbnailStrip.xaml(.cs)` | Row 0 scene-picker strip (`ItemsControl` over `Scenes`), staged-red / live-green border highlight |
+| `LeftPanel.xaml(.cs)` | Row 1 Col 0 left panel: edit mode → layers+properties; live → YouTube chat. Source add/edit/remove, list drag-to-reorder, snapshot revert-accept |
+| `OverlayHost.xaml(.cs)` | Root-grid overlay dialogs: Settings / Report Bug / Feature Request / About (+ licenses + license-key entry). `BackToAbout_Click` lives here |
+| `SceneThumbnailStrip.xaml.cs` | Code-behind for the strip above |
+| `LeftPanel.xaml.cs` | Code-behind for the panel above |
+
+## Window ↔ control contract
+
+The shell (`MainWindow.xaml(.cs)`) wires up each control and routes the few
+cross-cutting calls through it:
+
+- Ctor finds each control by `x:Name` (`TopBar`, `PreviewPane`, `LeftPanel`)
+  AFTER `InitializeComponent`, **before** any method touches it (null-ref guard).
+- `_previewPane.IsClickInsidePreview/Drawer` + `_leftPanel.IsClickInside` →
+  deselection guard in `Window_PreviewMouseLeftButtonDown`.
+- `_previewPane.UpdateSelectionOverlay(bool)` + `_leftPanel.OnSelectionChanged(vm)`
+  → selection/edit-mode routing from the VM's `SelectedElement` change.
+- `_topBar.RefreshAvatar()` on `IsConnected`/`AccountAvatarUrl` change.
+- Handlers in moved controls cast `(DataContext as MainViewModel)` — controls
+  inherit the window's DataContext; never pass the VM through a property.
+
+## Gotchas learned the hard way
+
+- **A UserControl cannot see `Window.Resources`** (StaticResource resolves by
+  element lookup, so it stops at the control boundary). The 7 converters
+  (`BoolToVis`, `InverseBoolToVis`, `NotNullToVis`, `NullToVis`, `HiddenBoolToVis`,
+  `EnumToBool`, `AllTrueToVis`) were promoted from `MainWindow.Resources` into
+  `Themes/Controls.xaml` (see [`Themes/index.md`](../Themes/index.md)).
+- **Moved XAML can lose its namespace**: a block cut from MainWindow carries no
+  root xmlns, so any `models:`/`Helpers:` ref in it breaks the build with
+  `MC2000: Value cannot be null (key)`. Declare `xmlns:models` / `xmlns:Helpers`
+  on each control's root (LeftPanel hit this).
+- **Name-looked-up host stubs must stay in the window**, not a control:
+  `WebViewHostPanel` (ctor `InitWebView2(WebViewHostPanel)`) and `ToastArea`
+  (`NotificationService.InArea("ToastArea")`) are resolved by name from the
+  window root — so `OverlayHost` deliberately leaves them as thin window-root
+  children (Commit 18).
+- When cutting visual wrappers, don't drop the outer `Border`'s
+  background/glow/margin — Commit 15 initially lost the preview's outer border
+  and had to restore it same-commit.
+
+Related: [`ViewModels/index.md`](../ViewModels/index.md) (MainViewModel the
+controls bind to), [`Themes/index.md`](../Themes/index.md) (styles/converters),
+[`Helpers/index.md`](../Helpers/index.md) (ViewModelBase, converters), the
+`MainWindow.xaml` shell at repo root that hosts these.