🐛 Clear PlyStackEntry shared array on return

[eduherminio]

Not clearing it causes an issue (non deterministic bench command results) when attempting to refactor TT initialization in #1150, when:
This issue appears when:

• TT initialization is moved outside of Engine.cs (i.e. to top level Program.cs in repro below, other class than engine in PR above)
• TT initialization uses pinned: true
• GC mode is Workstation

Better reproduced in #1181

Test  | bugfix/clear-PlyStackEntry-sharedarray-onreturn
Elo   | 4.76 +- 5.21 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.94 (-2.25, 2.89) [-5.00, 0.00]
Games | 7224: +2072 -1973 =3179
Penta | [157, 828, 1563, 887, 177]
https://openbench.lynx-chess.com/test/957/