Reference source: filtered Lichess open database (≥2400, Elite-style) for opening/move stats

[jozef2svrcek]

Not in current scope — logging for later consideration (came up during multi-source work, #40).

Import the Lichess open database (database.lichess.org standard dumps, CC0) but heavily filtered — e.g. both players rated ≥ 2400 — in the spirit of the third-party Lichess Elite Database.

Purpose: opening ideas + move statistics over a huge sample, not games browsing.

Notes / constraints:

• Raw dumps are enormous (~30 GB/month compressed, ~8B games total) and .pgn.zst. Must stream + filter on the fly; never import wholesale.
• Player names are anonymous/fantasy handles → skip FIDE lookup/normalization for this source.
• Standard chess only (no Chess960), so it won't reintroduce variants.
• Fits the multi-source framework (Support for more reference databases #40) as a streaming/bulk source with a rating filter. Likely wants its own "stats-only" treatment (positions index heavy; maybe not full game rows).