games/stockfish: Update 18 => 19

Summary of changes:
+ In tests against Stockfish 18, brings an Elo gain of up to 44 points
+ Wins more than three times as many game pairs as it loses
+ This release introduces the SFNNv16 network architecture, reducing binary size
+ Increased strength by introducing new pawn-pair features
+ The secondary neural network, introduced in Stockfish 16.1, has been retired
+ stricter validation for board positions, FEN strings, and UCI commands

Release Notes:
https://stockfishchess.org/blog/2026/stockfish-19/

Changelog:
https://github.com/official-stockfish/Stockfish/releases/tag/sf_19

PR: 298289
Approved by: osa, vvd (Mentors, implicit)