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/Composition/InteractionRetainedUiComposition.cs

@@ -650,7 +650,7 @@ internal sealed class RetailInteractionRetainedUiCompositionFactory
             // ResolveText several times per Refresh, and CharacterCreation-
             // UiController.ApplyProgressState forces a full refresh on
             // every page switch (`_lastRevision = long.MinValue`) — so an
-            // unchached resolver meant several fresh allocations + DatLock
+            // uncached resolver meant several fresh allocations + DatLock
             // acquisitions per click. DatStringResolver's own constructor
             // does no DAT I/O (only .Resolve reads), so building it here
             // outside the lock matches this file's existing pattern