fix(app,headless): Campaign CC slice CC4 re-review round — R1 arbiter + R2-R4

CC4 re-review returned NOT CLOSED: R1 (MEDIUM, blocking) is a new residual
the F1 fix itself introduced, plus three LOW riders (R2, R3, R4).

R1 — nulling UiRoot.FixedCanvasSize on chargen Close() stripped it from
character-management, which stays active underneath and only sets the
canvas on its own activation edge. Root cause (reviewer-named): two
controllers writing one host-global with no owner. Fixed with the
root-cause shape (reviewer's option (c)): UiRoot.DeclareFixedCanvas(owner,
size)/RevokeFixedCanvas(owner), an owner-scoped arbiter — every declarer
must agree on the canvas size (a mismatch throws instead of silently
last-writer-wins), and the canvas nulls only once EVERY declarer has
revoked. Both CharacterCreationUiController and CharacterManagementUi-
Controller now declare/revoke instead of writing FixedCanvasSize directly;
grepped for stragglers, none remain in production code (the raw setter
stays public only for UiRootFixedCanvasTests' isolated scale-math
coverage). New test (reviewer-specified):
CharacterScreensFixedCanvasArbiterTests — two controllers sharing one
UiRoot, proving the canvas stays set through chargen's Exit-confirm Close
while char-management is still active, nulling only once char-management
also deactivates, plus the original F1 defect's own covering case (both
revoke together at world entry).

R3 — HeadlessSessionHostTests.ContentLease_InstallsRealChargenOptions_
SelectHeritageIsAccepted proves F6's install actually opens the gate: a
content lease carrying a real hand-built DatCharGen heritage (not
ChargenOptions.Empty) is installed, and TrySelectHeritage for it succeeds.

R2 — filed docs/ISSUES.md #402 for the pre-existing
Streaming.LandblockBuildFactoryTests.Build_UsesTheSuppliedSharedReaderGate
full-suite flake (unrelated to Campaign CC).

R4 — fixed "unchached" -> "uncached" typo in
InteractionRetainedUiComposition.cs.

Runtime 1713/0, App 5127/13 skips (+2), Headless 166/0 (+1), full solution
Release build green. Live-DAT probes 7/7 under ACDREAM_PROBE_LIVE_MOUNT=1.
The known #402 flake did not fire across 3 consecutive full-suite runs
this session.

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

src/AcDream.App/UI/UiRoot.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Numerics;
 
 namespace AcDream.App.UI;
@@ -41,9 +42,94 @@ public sealed class UiRoot : UiElement
     /// 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.
+    ///
+    /// <para>
+    /// Campaign CC slice CC4 review-fix round R1 (2026-08-15): this raw
+    /// setter remains public for tests that exercise the scale/mouse-
+    /// mapping math in isolation (<c>UiRootFixedCanvasTests</c>), but
+    /// PRODUCTION code must go through <see cref="DeclareFixedCanvas"/>/
+    /// <see cref="RevokeFixedCanvas"/> instead of writing this property
+    /// directly. Two fixed-canvas screens can be active at once
+    /// (character-management underneath character-creation) and a raw
+    /// write from either one is a last-writer-wins race with no owner —
+    /// the F1 fix's own <c>Close()</c> null wiped the OTHER screen's still-
+    /// active canvas out from under it (see AD-98).
+    /// </para>
     /// </summary>
     public Vector2? FixedCanvasSize { get; set; }
 
+    /// <summary>Screens currently declaring a fixed canvas, keyed by owner
+    /// (see <see cref="DeclareFixedCanvas"/>).</summary>
+    private readonly Dictionary<object, Vector2> _fixedCanvasDeclarations = new();
+
+    /// <summary>
+    /// Declares that <paramref name="owner"/> wants the retained tree laid
+    /// out in <paramref name="size"/> while it is active. This is the single
+    /// arbiter for <see cref="FixedCanvasSize"/>: multiple owners may declare
+    /// concurrently (character-management stays declared while character-
+    /// creation is also open on top of it), and the effective
+    /// <see cref="FixedCanvasSize"/> is the shared declaration set's value.
+    /// Every current declarer must agree on the size — a mismatched second
+    /// declaration throws rather than silently overwriting the first
+    /// (Campaign CC CC4 review-fix round R1, 2026-08-15; see
+    /// <c>docs/architecture/retail-divergence-register.md</c> AD-98). Pair
+    /// every call with <see cref="RevokeFixedCanvas"/> on the SAME owner at
+    /// deactivate/close/dispose.
+    /// </summary>
+    public void DeclareFixedCanvas(object owner, Vector2 size)
+    {
+        ArgumentNullException.ThrowIfNull(owner);
+        if (_fixedCanvasDeclarations.TryGetValue(owner, out Vector2 existing))
+        {
+            if (existing == size)
+                return; // idempotent re-declare (e.g. a re-ticked activation edge)
+            throw new InvalidOperationException(
+                $"UiRoot.DeclareFixedCanvas: owner {owner} re-declared a different " +
+                $"canvas ({existing} -> {size}) without revoking first.");
+        }
+
+        foreach (Vector2 declared in _fixedCanvasDeclarations.Values)
+        {
+            if (declared != size)
+            {
+                throw new InvalidOperationException(
+                    $"UiRoot.DeclareFixedCanvas: owner {owner} declared {size} but " +
+                    $"another active owner already declared {declared} — every " +
+                    "concurrently-active fixed-canvas screen must author the SAME " +
+                    "canvas size (see AD-98).");
+            }
+        }
+
+        _fixedCanvasDeclarations[owner] = size;
+        FixedCanvasSize = size;
+    }
+
+    /// <summary>Revokes <paramref name="owner"/>'s declaration from
+    /// <see cref="DeclareFixedCanvas"/>. <see cref="FixedCanvasSize"/>
+    /// becomes null only once EVERY declarer has revoked; while another
+    /// owner is still declared, it stays set to that shared value. A
+    /// revoke from an owner that never declared (or already revoked) is a
+    /// no-op, matching the idempotent shutdown paths (<c>Deactivate</c>
+    /// AND <c>Dispose</c> can both revoke the same owner).</summary>
+    public void RevokeFixedCanvas(object owner)
+    {
+        ArgumentNullException.ThrowIfNull(owner);
+        if (!_fixedCanvasDeclarations.Remove(owner))
+            return;
+
+        if (_fixedCanvasDeclarations.Count == 0)
+        {
+            FixedCanvasSize = null;
+            return;
+        }
+
+        foreach (Vector2 declared in _fixedCanvasDeclarations.Values)
+        {
+            FixedCanvasSize = declared;
+            break;
+        }
+    }
+
     /// <summary>
     /// The coordinate space the retained tree currently lays out in: the fixed
     /// authored canvas while one is active, else the window itself. Anything