refactor(battlenode): name sender-only vs both-sides handshake checks (§D)

Behavior-preserving; 231 BattleNode tests green.

FrameDispatchContext.BothAfterReady() -> BothSidesAfterReady() (7 call sites). The
4 inline `SenderPhase == AfterReady` checks in TurnEndHandler/TurnEndFinalHandler now
read a new SenderIsAfterReady property. Both carry cross-referencing docs so the
Bot-arm (sender-only) vs PvP-arm (both-sides) distinction is explicit at the type.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/Dispatch/FrameDispatchContext.cs

@@ -26,9 +26,16 @@ internal sealed class FrameDispatchContext
         set { if (From is IHasHandshakePhase p && value is { } v) p.Phase = v; }
     }
 
-    /// <summary>Both participants have completed the handshake. Reads A/B (not From/Other) so the
-    /// result is identical regardless of which side sent the frame — matches legacy BothAfterReady.</summary>
-    internal bool BothAfterReady() =>
+    /// <summary>Just the SENDER has finished the handshake — says nothing about the opponent. The
+    /// Bot arms gate on this (the bot has no handshake phase of its own); contrast
+    /// <see cref="BothSidesAfterReady"/>, which the PvP arms require. The sender-only vs both-sides
+    /// distinction is load-bearing for the Bot/PvP split (see TurnEndHandler / TurnEndFinalHandler).</summary>
+    internal bool SenderIsAfterReady => SenderPhase == HandshakePhase.AfterReady;
+
+    /// <summary>BOTH participants have finished the handshake. Reads A/B (not From/Other) so the
+    /// result is identical regardless of which side sent the frame. Contrast
+    /// <see cref="SenderIsAfterReady"/> (sender only).</summary>
+    internal bool BothSidesAfterReady() =>
         (A as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady &&
         (B as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady;
 }