games/stockfish: Update 18 => 19HEADmain

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)

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