feat(net): L.3b — capture per-object friction + elasticity from CreateObject

Companion to L.3a (a1c27b3) which ported the velocity-reflection bounce.
Previously the CreateObject parser did `pos += 4` for both Friction and
Elasticity floats — silently dropping the wire data so every entity got
the PhysicsBody constructor default (0.05 elasticity, 0.5 friction).

Server-set bouncier surfaces or stickier objects therefore felt
identical to inert walls on collision. Inelastic projectiles via
PhysicsState bit 0x20000 (already plumbed in Commit A) had no per-
object elasticity to override.

Now the parser captures the floats, surfaces them on Parsed +
EntitySpawn, leaving the values at default (null) when their
PhysicsDescriptionFlag bits aren't set. Subscribers (e.g., the
remote-entity dead-reckoning path, future spell-projectile rendering)
can apply them when they wire elasticity to PhysicsBody.Elasticity.

The local player's PhysicsBody is constructed at controller init,
not from a CreateObject — so this commit alone produces no
user-visible local-player change. Effect lands when remote/projectile
physics consume EntitySpawn.Elasticity.

Files:
- CreateObject.cs:284-294: declare friction + elasticity accumulators.
- CreateObject.cs:467-487: parse floats instead of skipping.
- CreateObject.cs:543-555: propagate to Parsed via both return paths.
- WorldSession.cs:67-71: extend EntitySpawn record.
- WorldSession.cs:665-668: pipe through to subscribers.

Tests: 1491 still pass.

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

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

@@ -120,7 +120,13 @@ public static class CreateObject
         ushort ServerControlSequence = 0,
         ushort ForcePositionSequence = 0,
         uint? PhysicsState = null,
-        uint? ObjectDescriptionFlags = null);
+        uint? ObjectDescriptionFlags = null,
+        // L.3b (2026-04-30): per-object friction + elasticity from the
+        // wire. Default to null when their PhysicsDescriptionFlag bits
+        // weren't set; subscribers fall back to PhysicsBody constructor
+        // defaults (0.05f elasticity, 0.5f friction).
+        float? Friction = null,
+        float? Elasticity = null);
 
     /// <summary>
     /// The relevant subset of the server-sent <c>MovementData</c> /
@@ -286,6 +292,13 @@ public static class CreateObject
         // "ObjectDescriptionFlags" at the WeenieHeader trailer.
         uint? physicsState = null;
         uint? objectDescriptionFlags = null;
+        // L.3b (2026-04-30): per-object friction + elasticity. Wire-encoded
+        // when their PhysicsDescriptionFlag bits are set. Default values
+        // come from PhysicsBody constructors; these overrides drive the
+        // velocity-reflection bounce magnitude per object (e.g., bouncier
+        // platforms vs. inert walls).
+        float? friction = null;
+        float? elasticity = null;
 
         try
         {
@@ -453,8 +466,25 @@ public static class CreateObject
                 objScale = BinaryPrimitives.ReadSingleLittleEndian(body.Slice(pos));
                 pos += 4;
             }
-            if ((physicsFlags & PhysicsDescriptionFlag.Friction) != 0) pos += 4;
-            if ((physicsFlags & PhysicsDescriptionFlag.Elasticity) != 0) pos += 4;
+            if ((physicsFlags & PhysicsDescriptionFlag.Friction) != 0)
+            {
+                if (body.Length - pos < 4) return PartialResult();
+                friction = BinaryPrimitives.ReadSingleLittleEndian(body.Slice(pos));
+                pos += 4;
+            }
+            if ((physicsFlags & PhysicsDescriptionFlag.Elasticity) != 0)
+            {
+                // L.3b (2026-04-30): capture instead of skipping. The wire
+                // float is the per-object elasticity used by the velocity-
+                // reflection bounce (CPhysicsObj::set_elasticity at
+                // acclient_2013_pseudo_c.txt:277817, clamped to [0, 0.1]).
+                // Was previously dropped — every object got the default
+                // 0.05f, so server-set bouncier surfaces felt identical to
+                // walls.
+                if (body.Length - pos < 4) return PartialResult();
+                elasticity = BinaryPrimitives.ReadSingleLittleEndian(body.Slice(pos));
+                pos += 4;
+            }
             if ((physicsFlags & PhysicsDescriptionFlag.Translucency) != 0) pos += 4;
             if ((physicsFlags & PhysicsDescriptionFlag.Velocity) != 0) pos += 12;  // vec3
             if ((physicsFlags & PhysicsDescriptionFlag.Acceleration) != 0) pos += 12;
@@ -510,14 +540,18 @@ public static class CreateObject
             return new Parsed(guid, position, setupTableId, animParts,
                 textureChanges, subPalettes, basePaletteId, objScale, name, itemType, motionState, motionTableId,
                 instanceSeq, teleportSeq, serverControlSeq, forcePositionSeq,
-                physicsState, objectDescriptionFlags);
+                physicsState, objectDescriptionFlags,
+                friction, elasticity);
 
             // Local helper: if we ran out of fields past PhysicsData, still
             // return the useful prefix (guid/position/setup/animParts/textures/palettes/scale/motion).
             Parsed PartialResult() => new(
                 guid, position, setupTableId, animParts,
                 textureChanges, subPalettes, basePaletteId, objScale, null, null, motionState, motionTableId,
-                PhysicsState: physicsState, ObjectDescriptionFlags: objectDescriptionFlags);
+                PhysicsState: physicsState,
+                ObjectDescriptionFlags: objectDescriptionFlags,
+                Friction: friction,
+                Elasticity: elasticity);
         }
         catch
         {

src/AcDream.Core.Net/WorldSession.cs

@@ -63,7 +63,13 @@ public sealed class WorldSession : IDisposable
         // ObjectDescriptionFlags: retail PWD._bitfield (acclient.h:6431-6463)
         // — drives IsPlayer/IsPK/IsPKLite/IsImpenetrable for PvP gating.
         uint? PhysicsState = null,
-        uint? ObjectDescriptionFlags = null);
+        uint? ObjectDescriptionFlags = null,
+        // L.3b (2026-04-30): per-object physics tuning from the wire.
+        // Friction defaults to PhysicsBody constructor value (0.5f).
+        // Elasticity defaults to 0.05f. When set, drives the velocity-
+        // reflection bounce magnitude (clamped to [0, 0.1] retail-side).
+        float? Friction = null,
+        float? Elasticity = null);
 
     /// <summary>Fires when the session finishes parsing a CreateObject.</summary>
     public event Action<EntitySpawn>? EntitySpawned;
@@ -657,7 +663,9 @@ public sealed class WorldSession : IDisposable
                         parsed.Value.MotionState,
                         parsed.Value.MotionTableId,
                         parsed.Value.PhysicsState,
-                        parsed.Value.ObjectDescriptionFlags));
+                        parsed.Value.ObjectDescriptionFlags,
+                        parsed.Value.Friction,
+                        parsed.Value.Elasticity));
                 }
             }
             else if (op == DeleteObject.Opcode)