Return an actual firing sequence from the Gonality Game's verbose mode

[Copilot]

CFGonality._find_winning_sequence was a stub returning [], so play_gonality_game(..., verbose=True) reported Player A as the winner without ever showing how the debt was resolved. The broader gonality / treewidth / Dhar / Platonic-solid infrastructure called for in issue #6 is already in place; this fills the one concrete remaining placeholder in the gonality-game path.

Changes

• chipfiring/CFGonality.py — Implement _find_winning_sequence by reusing the existing DharAlgorithm in an EWD-style loop:
  • Pick q as the min-degree vertex (matching algo.EWD).
  • Iteratively run Dhar's burning algorithm to obtain a maximal legal firing set on V − {q}, fire it, and append the fired vertex names (sorted) to the returned sequence.
  • Operate on a copy of the divisor so the caller's input is never mutated.
  • Return [] for already-effective inputs and for non-winnable inputs (detected via residual debt at q after q-reduction).
  • Defensive O(|V|²) iteration cap to guarantee termination on pathological inputs.
• tests/test_cfgonality.py — New TestFindWinningSequence covering: already-effective input, debt resolution on K₃ and on a path graph (with end-to-end verification that applying the returned firings produces an effective divisor), non-winnable input, input non-mutation, and verbose play_gonality_game.

Example

from chipfiring import CFGraph, CFDivisor, play_gonality_game

g = CFGraph({"A", "B", "C"}, [("A", "B", 1), ("B", "C", 1), ("A", "C", 1)])
placement = CFDivisor(g, [("A", 2), ("B", 0), ("C", 0)])  # Player A's 2 chips on A
result = play_gonality_game(g, 2, placement, "B", verbose=True)  # Player B targets B

result.player_a_wins      # True
result.winning_sequence   # ['A']  -- previously always []