Add chess.chesstb: pure-Python prober for chesstb endgame tablebases

chesstb (https://github.com/noobpwnftw/chesstb) ships three table types per
material: 50-move-rule-aware WDL (with cursed/blessed classes), DTC
(distance-to-conversion), and a DTM50 pack that answers both the unbounded DTM
and the exact 50MR DTM at any halfmove clock.

This adds chess/chesstb.py, a faithful pure-Python re-implementation of the
reference C++ prober (no native extension; depends only on the standard
library's lzma plus a small bundled LZ4-block decoder). It covers the position
index (symmetry canonicalization, king/pawn slice managers, binomial
piece-group ranking, index-permutation layout), all three decoders, and the
probe orchestration (dropped-frame one-ply-minimax reconstruction for shrunk
files, the en-passant overlay, and DTM50 halfmove-clock layer selection).

API mirrors chess.syzygy: open_tablebase(); probe_wdl/probe_dtz/probe_dtm
return signed values; probe_dtm50 returns (wdl, plies); get_* are non-raising.

Validated bit-for-bit against the reference C++ prober across all 145 shipped
<=5-man materials and every halfmove-clock layer. A small fixture set
(data/chesstb) drives ChesstbTestCase.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: noobpwnftw