fix(phys L.2g slice 1b): widen CollisionExemption to ETHEREAL alone

B.4b visual test confirmed the L.2g slice 1 handoff's open question:
ACE's Door.Open() broadcasts state=0x0001000C (HasPhysicsBSP |
Ethereal | ReportCollisions), NOT the state=0x14+ that retail servers
send (Ethereal | IgnoreCollisions). The L.2g pipeline correctly
mutates ShadowObjectRegistry with the new state, but
CollisionExemption.ShouldSkip required both bits and the door stayed
solid.

Retail (acclient_2013_pseudo_c.txt:276782) wraps FindObjCollisions in
`if NOT (state & ETHEREAL && state & IGNORE_COLLISIONS)`. ETHEREAL
alone takes a different retail path at line 276795 that sets
sphere_path.obstruction_ethereal = 1 and lets downstream movement
allow passage despite the contact. We haven't ported that downstream
path yet.

Pragmatic shortcut: widen the early-out to ETHEREAL alone so doors
become passable when ACE flips the bit. Retail-server broadcasts
still hit the same branch correctly (both bits set implies ETHEREAL).
Compatible with both server styles.

Renames test EtherealOnly_NotSkipped -> EtherealOnly_Skipped and
flips its assertion. 13 CollisionExemption tests pass; full suite
1046 pass / 8 pre-existing baseline fail (unchanged).

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

src/AcDream.Core/Physics/CollisionExemption.cs

@@ -59,14 +59,24 @@ public static class CollisionExemption
     public static bool ShouldSkip(uint targetState, EntityCollisionFlags targetFlags,
                                    ObjectInfoState moverState)
     {
-        // 1. Target ETHEREAL + IGNORE_COLLISIONS → walk through.
+        // 1. Target ETHEREAL → walk through.
-        //    acclient_2013_pseudo_c.txt:276782 — wraps the entire body of
+        //    Retail (acclient_2013_pseudo_c.txt:276782) requires BOTH
-        //    FindObjCollisions; we hoist it as the first early-out.
+        //    ETHEREAL_PS (0x4) AND IGNORE_COLLISIONS_PS (0x10) to wrap
-        if ((targetState & ETHEREAL_PS) != 0
+        //    the entire body of FindObjCollisions and skip collision.
-            && (targetState & IGNORE_COLLISIONS_PS) != 0)
+        //    ETHEREAL alone takes a different retail path (line 276795
-        {
+        //    sets sphere_path.obstruction_ethereal = 1 and downstream
+        //    movement allows passage despite the contact). We haven't
+        //    ported that downstream path yet.
+        //
+        //    L.2g slice 1b (2026-05-13): ACE's Door.Open() sends only
+        //    ETHEREAL (state=0x0001000C observed live), not the
+        //    ETHEREAL|IGNORE_COLLISIONS combo retail servers broadcast.
+        //    Pragmatic shortcut: exempt on ETHEREAL alone so doors
+        //    become passable when ACE flips the bit. Retail-server
+        //    broadcasts (state=0x14+) still hit this branch correctly
+        //    because both bits set implies ETHEREAL set.
+        if ((targetState & ETHEREAL_PS) != 0)
             return true;
-        }
 
         // 2. Viewer mover + creature target → walk through.
         //    acclient_2013_pseudo_c.txt:276787-276790.

tests/AcDream.Core.Tests/Physics/CollisionExemptionTests.cs

@@ -42,12 +42,16 @@ public class CollisionExemptionTests
     }
 
     [Fact]
-    public void EtherealOnly_NotSkipped()
+    public void EtherealOnly_Skipped()
     {
-        // Target with ETHEREAL but NOT IGNORE_COLLISIONS does not bail
+        // L.2g slice 1b (2026-05-13): ETHEREAL alone exempts collision.
-        // out at the first gate — collision proceeds. (Step-down marks
+        // Retail (acclient_2013_pseudo_c.txt:276782) required both bits,
-        // obstruction_ethereal, but does not exempt.)
+        // but ACE's Door.Open() broadcasts ETHEREAL alone — observed
-        Assert.False(CollisionExemption.ShouldSkip(
+        // live: state=0x0001000C (HasPhysicsBSP | Ethereal | ReportCollisions).
+        // Pragmatic shortcut: widen the early-out to ETHEREAL alone so
+        // doors become passable when ACE flips the bit. Retail-server
+        // broadcasts (state=0x14+) still hit the same branch correctly.
+        Assert.True(CollisionExemption.ShouldSkip(
             targetState: ETHEREAL_PS,
             targetFlags: EntityCollisionFlags.None,
             moverState: ObjectInfoState.IsPlayer));