Fix opening explorer labels after recentering

[Moltenfire]

The opening explorer recomputed notation after hypothetical moves, but those moves can recenter the cloned board and leave the original target coordinate stale.

That stale coordinate could produce invalid or self-referential move labels, or panic the client. Reuse the notation recorded before re-centering and cover the boundary with a regression test.

Found during an additional AI-assisted review of the PR.

Example

Concrete example (from the added regression test):

• Board has White queen/ant near one edge (q=30/31) and Black queen/ant near the other (q=0/1) on the same row.
• Playing wG1 to Position(30, 15) is legal, but triggers the board's internal re-centering.
• After the move, state.board.position_of_piece(wG1) is no longer (30, 15) — it's been shifted.
• Old code: recomputed the label using the stale (30, 15) coordinate against the re-centered board → wrong/garbage label.
• New code: instead reuses the notation that was already recorded correctly in state.history.moves at the time the move was actually played (before any subsequent re-centering could invalidate it) → always correct.