feat(runtime): construct remote bodies at Create per retail set_description

Cutover slice C3b: residence-route remote/creature/projectile Creates now
get their canonical PhysicsBody at Create time — retail's order, closing
the C3 flip's Finding C (production builds bodies at first motion; retail
builds them in ACCObjectMaint::CreateObject). RuntimeRemoteBodyDescription
walks set_description 0x00514F40 exactly: the motion-table gate (zero id
PASSES — verified at 0051871f/005127ca), the frame-vs-movement branch
keyed on retail's movement_buffer != 0 (an empty-buffer movement payload
takes the PLACEMENT branch and writes no autonomy — the wire-shape defect
the retail review caught), set_state, the byte-certain friction gate
(inclusive [0,1]; NaN deliberately skipped per the gates doc's sanctioned
deviation), the set_elasticity clamp with retail's unordered-to-zero NaN
routing (ACE diverges to 0.1 on that edge), the translucency gate
(!= 0.0f, original always recorded), velocity via setter, omega raw, and
ctor-defaults for absent wire fields (0.95f/0.05f/0 — the fresh-desc-per-
message flow verified at both UnPack call sites). InWorld stays false
until submission, the enter_world analog.

RuntimeRemoteFirstEntryState sequences mover-prep -> body construction ->
placement -> acknowledgement -> Execute with every C3a hardening
inherited: exactly-once stages, the shared acknowledge-stage
discriminator (extracted to RuntimeFirstEntryAcknowledgement, one body
for both conductors), typed Contention against in-flight remote-motion
binds, never-clobber body binding through the canonical writer (foreign
body fails closed — provably safe coexistence with today's
build-at-first-motion path in both directions), automatic convergence
through the retirement fan-out, and the construction receipt riding the
terminal Advance. Dormant: no production caller; C3c wires both hosts.

Reviewed: retail-conformance PASS (the construction order, both gate
boundary/NaN semantics, the motion-table and autonomy verdicts all
re-derived from the pseudo-C) + architecture/adversarial PASS after one
fix round. Runtime 982/982; complete Release solution 10,777 passed / 4
intentional skips.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Runtime/Gameplay/RuntimeLocalPlayerFirstEntryState.cs

@@ -533,49 +533,26 @@ internal sealed class RuntimeLocalPlayerFirstEntryState
     }
 
     /// <summary>
-    /// Re-validates authority after a failed acknowledge. Two independent
-    /// checks, either of which failing means authority moved and this class
-    /// must abandon rather than keep retrying forever: (1) the residence
-    /// lease this whole sequence began under must still be exactly current
-    /// (mirrors the stage0/stage1 checks — a delete or reset retires it);
-    /// (2) if the FIFO head belongs to THIS entity at all, it must still be
-    /// the exact Place projection this class is holding — a head that
-    /// belongs to us but is no longer that exact token (rewritten to
-    /// Discard, or to a later revision) means our specific placement was
-    /// superseded even if the residence lookup transiently still resolves.
-    /// A head belonging to a DIFFERENT entity is the genuine "not yet our
-    /// turn" case and must stay retryable. This reads the Runtime-internal
-    /// <see cref="RuntimeSetPositionState.TryPeekProjection"/> directly
-    /// rather than through the public, generation-gated
-    /// <see cref="RuntimePlacementProjectionChannel"/> — this class is part
-    /// of Runtime, not an external host crossing that boundary, exactly like
-    /// its existing direct <see cref="RuntimeSetPositionState.AcknowledgeProjection"/>
-    /// call above.
+    /// Re-validates authority after a failed acknowledge. C3b review M2:
+    /// the mechanism (residence-lease currency + exact-head-token match; a
+    /// DIFFERENT entity's head stays retryable) is shared verbatim with the
+    /// remote conductor via
+    /// <see cref="RuntimeFirstEntryAcknowledgement.IsStillPending"/> — one
+    /// body, so the abandonment fix that stops a delete-rewritten Discard
+    /// head from producing an infinite AwaitingReceiptAcknowledgement retry
+    /// cannot regress independently in either conductor. Full rationale on
+    /// the shared helper.
     /// </summary>
     private bool IsAcknowledgementStillPending(
         RuntimeEntityRecord record,
         in RuntimeInitialCreateResidenceToken residenceToken,
-        in RuntimePlacementProjectionToken expected)
-    {
-        if (!_residences.TryGetCurrent(
-                record,
-                out RuntimeInitialCreateResidenceLease lease)
-            || lease.Token != residenceToken)
-        {
-            return false;
-        }
-
-        if (_physics.SetPosition.TryPeekProjection(
-                out RuntimePlacementProjectionSnapshot head)
-            && head.Token.Entity == expected.Entity
-            && (head.Kind is not RuntimePlacementProjectionKind.Place
-                || head.Token != expected))
-        {
-            return false;
-        }
-
-        return true;
-    }
+        in RuntimePlacementProjectionToken expected) =>
+        RuntimeFirstEntryAcknowledgement.IsStillPending(
+            _residences,
+            _physics.SetPosition,
+            record,
+            residenceToken,
+            expected);
 
     private RuntimeLocalPlayerFirstEntryStatus RunExecute(
         RuntimeEntityRecord record,