Add Syzygy support for giveaway/suicide/atomic variants

[Belzedar94]

Summary

• add a variant-aware Position::set overload and salt materialKey with the active Variant*, so Syzygy keys differentiate chess, giveaway/suicide, atomic, etc.
• introduce VariantTBConfig metadata (suffixes, pawnless suffixes, magic bytes) plus a VariantConfigMap that maps each registered Variant* to the appropriate config at startup
• enumerate tablebases for chess, giveaway/antichess, suicide, and all atomic variants (atomic, nocheckatomic, atomar), using the new config map rather than the old all-variants loop
• load tables with the correct classical Stockfish filename (KRvK, etc.) and map to PieceToChar via the variant’s template when the immutable piece order is needed
• port MV-Stockfish’s antichess “threat” probing (sprobe_ab/sprobe_captures) and introduce a THREAT probe state so giveaway/suicide DTZ behaves like upstream; atomic keeps its connected-king tweaks
• wire up a runtime suffix/loader that opens .gtbw/.stbw/.atbw and their pawnless companions, using the variant-specific magic numbers in mapped()

Details

• build the engine normally (ninja or make build)
• run lints/CI as usual
• spot-check table probing:
  1. load the engine with Syzygy paths pointing to standard .rtbw/.rtbz → verify chess probing still works
  2. place a giveaway/antichess table position (e.g., KRvK with .gtbw) → confirm WDL/DTZ results match MV-SF
  3. place a suicide position (*.stbw) → verify THREAT and DTZ output
  4. place an atomic position (*.atbw) and ensure the connect‑kings logic still matches expectations

[Belzedar94]

@codex , the bench changed because you modified the materialKey to make it variant-specific. This could potentially make sense since the TB probing uses that. However, there are two problems with that:

1. The material key absolutely has to be deterministic
2. Instead of hacking something together with the variant-tablebase association, it should have something similar as for the endgame code (EndgameEval) and basically hardwire the variant to tablebase association using a variant object property.

[chatgpt-codex-connector]

Summary

• Added a TablebaseVariant enum and per-variant property so each variant can declare its associated tablebase configuration explicitly. src/types.hL321-L332 src/variant.hL168-L227
• Tagged the chess, giveaway/antichess/suicide, and atomic-family variants with their tablebase variants to drive Syzygy integration. src/variant.cppL48-L52 src/variant.cppL392-L425 src/variant.cppL500-L530
• Generated deterministic Zobrist keys for tablebase variants and folded them into the material key instead of XORing raw variant pointers. src/position.cppL42-L50 src/position.cppL184-L189 src/position.cppL672-L673
• Updated the Syzygy loader to consult the new variant property when registering supported tablebases. src/syzygy/tbprobe.cppL105-L120 src/syzygy/tbprobe.cppL1613-L1617

Testing

• ✅ make -C src build ARCH=x86-64-modern

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