Reorder PieceKind discriminants: Bishop=5, Rook=6, Gold=7

[Copilot]

Aligns PieceKind representation with YaneuraOu's discriminant ordering, moving Gold after Bishop and Rook.

Breaking Changes

PieceKind discriminants reordered:

• Bishop: 6 → 5
• Rook: 7 → 6
• Gold: 5 → 7

Impact on serialization:

• USI piece symbols arrays updated to match new discriminant order
• Hand piece output order changes from RBGSNLPrbgsnlp to GRBSNLPgrbsnlp
• All array-indexed operations maintain correctness via array_index() method

Implementation

• piece_kind.rs: Enum discriminants, promote()/unpromote() match arms, all() ordering, USI symbols PLNSBRGK
• piece.rs: USI symbols PLNSBRGKplnsbrgk
• hand.rs: USI pieces array PLNSBRG, documentation updated

Binary representations and array indices shift for Gold/Bishop/Rook. Code depending on specific discriminant values will break.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Representation of PieceKind</issue_title>
<issue_description>Gold = 7, Bishop = 5, Rook = 6 style has many benefits, but it's a breaking change.
Ref: https://github.com/yaneurao/YaneuraOu/blob/v7.50-wcsc32/source/types.h#L461-L494

Changes in implementation:

• PieceKind
• impl ToUsi for PieceKind
• impl ToUsi for Piece
• impl ToUsi for [Hand; 2]
  </issue_description>

<agent_instructions>Can you try creating a PR that achieves the goal? I want to see what it looks like.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Representation of PieceKind #21

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[koba-e964]

@copilot did you read the rule file in the repo?

[Copilot]

@copilot did you read the rule file in the repo?

I searched for rule files in the repository but couldn't locate a specific rule file. Could you point me to which file you're referring to? I want to make sure I understand any conventions or requirements I may have missed.

The changes I made follow the issue description which requested Gold = 7, Bishop = 5, Rook = 6 and references the YaneuraOu types.h file. If there's a specific ordering convention or rule I should be following, I'd be happy to review and adjust the implementation accordingly.

[koba-e964]

@copilot It's located at:
https://github.com/rust-shogi-crates/shogi_core/blob/main/.github%2Fagents%2Fgithub-actions-pinning-rules.md

[Copilot]

@copilot It's located at:
https://github.com/rust-shogi-crates/shogi_core/blob/main/.github%2Fagents%2Fgithub-actions-pinning-rules.md

I've now read .github/agents/github-actions-pinning-rules.md.

However, this file contains rules about GitHub Actions workflow pinning conventions (using version tags for official actions and SHA pinning for third-party actions). It doesn't appear to relate to the PieceKind representation changes I made.

The changes in this PR modify PieceKind enum discriminants, ToUsi implementations, and related code - none of which involve GitHub Actions workflows. Could you clarify what aspect of this rule file should apply to the PieceKind representation changes? Or is there a different rule file you'd like me to review?