fix(render): branch inside/outside on is_player_outside, not the camera cell (PARTIAL)

Retail SmartBox::RenderNormalMode (0x453aa0:92665) branches DrawInside vs the
outdoor LScape::draw on is_player_outside (the PLAYER's cell, 0x451e80), then
roots DrawInside at the VIEWER cell. acdream keyed the whole branch off the
camera cell (clipRoot = visibility.CameraCell), so a 3rd-person chase camera
lagging in a doorway AFTER the player stepped outside took the DrawInside path
rooted at the threshold cell, where the exit-portal flood degenerates: grey
world + entities-through-walls. Now ShouldRenderIndoor(playerCellId,
viewerCellResolved) gates the branch on the player; the DrawInside root stays
the viewer cell (handoff invariant preserved).

SCOPE / HONESTY: this REDUCES the player-OUTSIDE doorway grey (visual-confirmed
reduced a lot) but does NOT fix the deeper symptom: when the player is in one
interior cell (cellar 0174) and the camera is in another (room 0171), the flood
roots at the camera cell and does NOT seal the player's cell, so the cellar
floor / interior walls drop to grey. That is the KNOWN R1-completion problem
(2026-06-05 Residual A handoff + 2026-06-02 design doc section 3: a
SHELL-SEALING / wrong-flood-root bug), not this branch.

Tests: Core 1331p / 4f (documented) / 1s, App 187p, build green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -272,4 +272,29 @@ public static class RenderingDiagnostics
     /// in the 8×8 landblock grid (0x0001–0x0040).
     /// </summary>
     public static bool IsEnvCellId(ulong id) => (id & 0xFFFFu) >= 0x0100u;
+
+    /// <summary>
+    /// The top-level render branch: should this frame run the indoor (DrawInside) path?
+    ///
+    /// <para>Retail <c>SmartBox::RenderNormalMode</c> (0x453aa0, pc:92665) branches
+    /// DrawInside vs the outdoor <c>LScape::draw</c> on <c>is_player_outside</c> — the
+    /// <b>PLAYER's</b> cell (<c>(player-&gt;m_position.objcell_id &amp; 0xFFFF) &lt; 0x100</c>,
+    /// <c>SmartBox::is_player_outside</c> 0x451e80) — NOT the camera/viewer cell. When the
+    /// player is inside it then roots the flood at the <b>viewer</b> cell
+    /// (<c>this-&gt;viewer_cell</c>). So the inside/outside <i>decision</i> follows the player;
+    /// only the indoor <i>root</i> follows the camera.</para>
+    ///
+    /// <para>acdream historically branched on the camera cell (a non-null
+    /// <c>visibility.CameraCell</c>). A 3rd-person chase camera lags the player, so when the
+    /// player had already stepped outside but the camera still sat in the doorway, the camera
+    /// branch wrongly chose DrawInside rooted at the doorway cell, where the exit-portal flood
+    /// degenerates → the whole static world (terrain + shells) gated off → grey screen with
+    /// only entities (which bypass the gate) showing through. Branching on the player removes it.</para>
+    ///
+    /// <param name="playerCellId">The player's current cell id (0 if unresolved → outside).</param>
+    /// <param name="viewerCellResolved">Whether a viewer/camera cell is available to root
+    /// DrawInside at. Indoor render needs both: the player inside AND a cell to root at.</param>
+    /// </summary>
+    public static bool ShouldRenderIndoor(uint playerCellId, bool viewerCellResolved)
+        => viewerCellResolved && IsEnvCellId(playerCellId);
 }