Refactor: rename methods/attrs/locals to follow Python conventions

[dbqpdb]

A focused rename pass to bring the codebase in line with Python naming conventions. Distinct from #45 (PEP 8 layout / whitespace / line length) — this issue is specifically about what things are named, while #45 is about how the code is formatted. There is some overlap on case style (camelCase → snake_case).

Scheduling note: this work is deferred until the following rlm.py-touching PRs land, to avoid heavy rebases:

• Quick bug bag: six independent small fixes (#29, #30, #31, #32, #33, #36) #49 (quick bug bag)
• Type hints for Board + Move; mypy CI workflow (advisory) (#46) #54 (type hints, Board+Move)
• Inline docs: docstrings + comment cleanup for Board + Move (#47) #55 (docstrings, Board+Move)
• Types + docs follow-up: Player/Game/GameController/Piece subclasses (#46, #47) #56 (types + docs, Player / Game / Piece subclasses)
• Small cleanups: dead code, version string, Loudmouth regex (#40, #41, #28) #57 (small cleanups)

Once those merge, this should be doable as one focused PR.

Scope

Four categories:

1. camelCase / mixedCase → snake_case for methods, attributes, and locals (PEP 8 for callables).
2. Privacy conventions — single underscore prefix (_foo) for methods and attributes that are clearly internal helpers.
3. Class-name cleanup — KQRBN_Piece has an underscore; PEP 8 says CapWords with no underscores.
4. Factory-method naming convention — classmethods that construct an instance should use the from_X form (e.g. Board.from_fen).

Concrete renames

Public method names (snake_case)

Current	Proposed
Board.isValidFENboard	Board.is_valid_fen_board
Board.is_FEN	Board.is_fen
Board.to_FEN_board	Board.to_fen_board
Board.convert_FEN_to_board_array	Board.fen_to_array (classmethod returning np.ndarray — not a Board factory, so not from_fen)
Board.squareIdx_to_boardIdxs	Board.square_idx_to_board_idxs

Privacy renames (only-called-internally → _-prefixed)

Currently public but only called from within their own class:

Current	Proposed
KQRBN_Piece.get_single_move	KQRBN_Piece._single_move
KQRBN_Piece.get_ray_moves	KQRBN_Piece._ray_moves
King.get_kingside_castle_move	King._kingside_castle_move
King.get_queenside_castle_move	King._queenside_castle_move
King.get_castling_allowed_by_position	King._castling_allowed_by_position
King.get_castling_allowed_by_check	King._castling_allowed_by_check

If any of these turn out to have external callers we haven't identified, that's a sign the privacy choice was wrong and they should stay public.

Class name

Current	Proposed
KQRBN_Piece	KQRBNPiece (or NonPawnPiece if we prefer descriptive over abbreviated — open to discussion)

Local variable names (camelCase → snake_case)

Mostly in Board.convert_FEN_to_board_array and Board.squareIdx_to_boardIdxs:

Current	Proposed
squareIdx	square_idx
fileIdx	file_idx
rankIdx	rank_idx
FEN_board, FEN_chars	fen_board, fen_chars
K_str (in find_king_square)	king_char

Already correct — no change needed

• All dunder methods (__init__, __getitem__, etc.)
• Class-level constants (EMPTY_SQUARE, FILE_TO_IDX_DICT, IDX_TO_FILE_DICT, PIECE_TO_FIGURINE_DICT, VALID_BOARD_SQUARE_CONTENTS_PATTERN)
• The Game, Board, Move, Piece, Player, King, Queen, Rook, Bishop, Knight, Pawn, Lexicon, Loudmouth, GameController, RLMPlayer, HumanPlayer, NRLMPlayer, TestRLM class names
• Module-level __version__ (after Cleanup: store versionNumber as a string, not a float #41 / Small cleanups: dead code, version string, Loudmouth regex (#40, #41, #28) #57 land)

Method-role tagging (@classmethod vs @staticmethod vs instance)

The current classmethod vs instance method split is mostly sensible. Two minor calls worth considering during the pass:

• Move.square_to_string(cls, sq) uses cls.IDX_TO_FILE_DICT. Could be @staticmethod if we hoisted IDX_TO_FILE_DICT out of the class, but cleaner as-is.
• Board.square_rank_str, Board.square_file_lett: similar — use Board.IDX_TO_FILE_DICT, fine as classmethods.

Reviewer judgement at the time, but defaulting to "leave as-is unless there's a reason."

Test plan

• All TestRLM cases pass (pytest after Test infra: pytest migration + tests / lint workflows (#23, #43, #44) #50 lands).
• A grep for the old names returns zero results in rlm.py, demo_splash.py, and tests/.
• import rlm still succeeds with no warnings.

Out of scope

• PEP 8 formatting (line length, whitespace, blank lines) — that's Cleanup: bring all code up to PEP 8 #45.
• The versionNumber rename — already in PR Small cleanups: dead code, version string, Loudmouth regex (#40, #41, #28) #57.
• The bigger-question renames like single_char (sometimes the piece glyph, sometimes the piece letter) — those are best left to a content-aware refactor, not a mechanical rename pass.