fix(ui): Campaign LA gate round 2 — character-select scales as one authored canvas

Third iteration on the screen, completing AD-98. The previous substitution
stretched only the root BACKGROUND while the child widgets stayed at their
authored 800x600 pixel positions - and the background painting carries
visual anchors (the World/Characters captions are art), so the user gate
showed captions overlapping the listbox and every widget misaligned
against the stretched art.

Retail model (established at 71bf24fb): fixed-canvas pre-world screens
render at authored 800x600 and the whole composed frame stretches once at
presentation; the blitter has no stretch mode. Our equivalent now does the
same one stage earlier:

- UiRoot.FixedCanvasSize: while the char-select screen is active, the
  retained tree lays out in its authored canvas and Draw scopes a uniform
  scale onto TextRenderer.CanvasScale; the mouse entry points apply the
  exact inverse so MouseX/MouseY and every hit test live in canvas space.
- TextRenderer.AppendQuad is the single emission chokepoint - sprites,
  rects, AND glyphs scale together, including retail-authentic non-uniform
  aspect distortion and stretched text. World-space HUD stays native (the
  scale resets outside UiRoot.Draw).
- CharacterManagementUiController stops resizing Root to the viewport;
  activate/deactivate/dispose set and clear the host canvas.
- UiDatElement returns to retail-pure copy-or-tile; the interim
  StretchOwnBackgroundToFill flag is deleted.
- AD-98 updated to describe the completed substitution.

Tests: canvas-scale quad math, inverse input mapping (window click lands
on the canvas-space widget), degenerate-size guards, controller keeps
authored extent + sets/clears the canvas. App suite 5085/6 skips; live-DAT
char-select probes 3/3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/UI/UiRoot.cs

@@ -32,6 +32,32 @@ public sealed class UiRoot : UiElement
     /// <summary>Single owner for named retained-window lifecycle and raise policy.</summary>
     public RetailWindowManager WindowManager { get; }
 
+    /// <summary>
+    /// Campaign LA gate round 2 (register AD-98): when set, the retained tree
+    /// is laid out in this fixed authored canvas (the char-select screen's
+    /// 800×600) and the whole tree — widgets, glyphs, art — is stretched to
+    /// the window as one unit, matching retail's present-time frame stretch
+    /// for fixed-canvas pre-world screens. Draw applies the scale at the
+    /// renderer's quad chokepoint; the mouse entry points apply the inverse,
+    /// so <see cref="MouseX"/>/<see cref="MouseY"/> and every hit test live
+    /// in canvas space. Null (the in-world default) is native 1:1.
+    /// </summary>
+    public Vector2? FixedCanvasSize { get; set; }
+
+    /// <summary>Window→canvas stretch factor; One when no fixed canvas is set.</summary>
+    public Vector2 CanvasScale =>
+        FixedCanvasSize is { X: > 0f, Y: > 0f } canvas && Width > 0f && Height > 0f
+            ? new Vector2(Width / canvas.X, Height / canvas.Y)
+            : Vector2.One;
+
+    private (int x, int y) MapWindowToCanvas(int x, int y)
+    {
+        Vector2 scale = CanvasScale;
+        return scale == Vector2.One
+            ? (x, y)
+            : ((int)MathF.Round(x / scale.X), (int)MathF.Round(y / scale.Y));
+    }
+
     // ── Device-level state ───────────────────────────────────────────────
     public int MouseX { get; private set; }
     public int MouseY { get; private set; }
@@ -370,6 +396,21 @@ public sealed class UiRoot : UiElement
     }
 
     public void Draw(UiRenderContext ctx)
+    {
+        // AD-98 fixed-canvas stretch: scope the renderer's canvas scale to
+        // exactly this tree's draws (world-space HUD stays native).
+        ctx.TextRenderer.CanvasScale = CanvasScale;
+        try
+        {
+            DrawCore(ctx);
+        }
+        finally
+        {
+            ctx.TextRenderer.CanvasScale = Vector2.One;
+        }
+    }
+
+    private void DrawCore(UiRenderContext ctx)
     {
         // Render children (panels) sorted by z-order — modal last so it
         // sits on top.
@@ -401,6 +442,7 @@ public sealed class UiRoot : UiElement
 
     public void OnMouseMove(int x, int y)
     {
+        (x, y) = MapWindowToCanvas(x, y);
         int dx = x - MouseX;
         int dy = y - MouseY;
         MouseX = x;
@@ -552,6 +594,7 @@ public sealed class UiRoot : UiElement
 
     public void OnMouseDown(UiMouseButton btn, int x, int y, uint flags = 0)
     {
+        (x, y) = MapWindowToCanvas(x, y);
         MouseX = x; MouseY = y;
         UpdateButtonFlag(btn, down: true);
         _pressX = x; _pressY = y;
@@ -707,6 +750,7 @@ public sealed class UiRoot : UiElement
 
     public void OnMouseUp(UiMouseButton btn, int x, int y, uint flags = 0)
     {
+        (x, y) = MapWindowToCanvas(x, y);
         MouseX = x; MouseY = y;
         UpdateButtonFlag(btn, down: false);