feat(B.7): retail-faithful target indicator via Setup.SelectionSphere

Replaces the mesh-AABB approximation with retail's actual selection
mechanism. The user observed the indicator was too small and didn't
scale with the object the way retail does — root cause was using the
wrong source data.

Retail trace (decomp anchors in named-retail/acclient_2013_pseudo_c.txt):
- VividTargetIndicator::Draw at 0x004f6c30 is registered as the
  SmartBox targetting callback (0x004f6df6).
- SmartBox::DoTargettingChecks at 0x00453bb4 calls
  SmartBox::GetObjectBoundingBox (0x00452e20) to compute the rect.
- GetObjectBoundingBox uses CPhysicsObj::GetSelectionSphere
  (0x0050ea40) → CPartArray::GetSelectionSphere (0x00518b80) which
  reads setup->selection_sphere from the DAT, applies part-array
  scale (component-wise on center, Z-scale on radius), then calls
  Render::GetViewerBBox (0x0054b400) to project the sphere as a
  screen-space camera-aligned BBox.
- VividTargetIndicator::OnDraw at 0x004f62b0 inflates that rect by
  one triangle width/height on every side before drawing (eax_21 /
  eax_23 in 0x004f6a0b–0x004f6a99), so the corner triangles sit
  outside the projected sphere with a small gap.

Implementation:
- GameWindow.TryGetEntitySelectionSphere reads setup.SelectionSphere
  from the DAT (Setup type already exposes Origin + Radius),
  applies entity scale, rotates center via entity orientation, and
  produces a world-space sphere.
- TargetIndicatorPanel.TryComputeScreenRectFromSphere projects the
  sphere center via the view-projection matrix and computes
  screenRadius = worldRadius * projection.M22 * viewport.Y /
  (2 * clip.W). M22 = cot(fovY/2) for a standard right-handed
  perspective. Mathematically equivalent to retail's
  Render::GetViewerBBox followed by 2-corner xformPointInternal,
  faster (no double projection).
- TargetInfo carries WorldSphereCenter + WorldSphereRadius (replaces
  the previous WorldAabbMin/Max). Fallback to per-type height
  heuristic still in place if Setup has no baked selection_sphere
  (rare; Radius <= 1e-4f short-circuits).
- Inflate by TriangleSize on every side matches retail's eax_21 +
  eax_23 offsets exactly.
- Triangle right-angle apex flipped to point INWARD toward the
  target (per user feedback) — apex at corner + (±t, ±t),
  hypotenuse along the outer diagonal of the corner.
- TriangleSize 10 → 14 → 8 (retail sprite is small).

Also fixes a parser bug in CreateObject.cs introduced in 58e1556:
BF_INCLUDES_SECOND_HEADER is 0x04000000 per acclient.h:6458 (ACE
ObjectDescriptionFlag.IncludesSecondHeader matches), NOT 0x80000000.
The wrong bit meant the weenieFlags2 4-byte skip never fired for
entities that had the bit set, potentially shifting Useability /
UseRadius reads by 4 bytes. Now correct.

Visual verification (2026-05-16):
- Holtburg town sign — indicator traces the visible sign + pole at
  the right size (matches retail screenshot proportions).
- Sign R-key still silent no-op (B.8 useability gate intact).
- NPCs / doors / items still get correctly-sized indicators.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/Messages/CreateObject.cs

@@ -537,8 +537,14 @@ public static class CreateObject
             float? useRadius = null;
             try
             {
+                // BF_INCLUDES_SECOND_HEADER = 0x04000000 per acclient.h:6458
+                // (ACE ObjectDescriptionFlag.IncludesSecondHeader matches).
+                // Earlier code had this as 0x80000000 — wrong bit, so the
+                // weenieFlags2 4-byte skip never fired for entities that
+                // actually had it set, corrupting downstream optional-tail
+                // offsets. Now correct.
                 bool hasSecondHeader = objectDescriptionFlags.HasValue
-                    && (objectDescriptionFlags.Value & 0x80000000u) != 0;
+                    && (objectDescriptionFlags.Value & 0x04000000u) != 0;
                 if (hasSecondHeader && body.Length - pos >= 4) pos += 4;  // weenieFlags2
 
                 if ((weenieFlags & 0x00000001u) != 0)         // PluralName