v1.0.6: the 1500s rules easter egg

Chess v1.0.6

Chess v1.0.6 adds a second easter egg: play chess as it stood around 1500, before castling and
the double pawn step existed. It is off by default and changes nothing unless switched on.

Added

The 1500s rules (easter-egg.enable-1500s-rules)

• No castling of any kind. This also turns vertical castling off: if both easter eggs are
  enabled, neither form of castling is allowed.
• Pawns advance one square only. There is no double first step.
• No en passant. With no double step, a pawn can never pass another.
• The King's Leap. Once per game, each king may make a single knight move. It is otherwise
  an ordinary king move: it may capture, but it may not land on its own pieces, next to the
  enemy king, or anywhere that leaves the king in check.

Premoves follow the same rules. Castling is never offered as a premove, and the King's Leap is
premovable for as long as it is unspent.

CPU opponents (/chess duelcpu) play by these rules too, including the King's Leap, and know
when theirs has been used.

Configuration

settings.yml gains one key under easter-egg:

easter-egg:
  enable-vertical-castling: false
  enable-1500s-rules: false

Existing installs need no editing. Chess adds missing keys from the bundled defaults at startup,
so the new key appears automatically, switched off.

Known limitation

The castled.codes web board suggests moves using standard chess rules. Under the 1500s rules the
server still decides what is legal, so it refuses a double pawn step or castling played from the
web, and a web player cannot make the King's Leap. In-game boards and CPU opponents are
unaffected.

Commands

/chess duel <player> <time>
/chess duelcpu <1-10> <time>
/chess accept <player>
/chess decline <player>
/chess watch <player>
/chess open
/chess help

Commands are unchanged from v1.0.5.

Verification

• gradle clean build succeeds, with 82 tests passing.
• The 1500s rules are covered for castling (including when vertical castling is also on),
  single-step pawns, en passant, the King's Leap's destinations, captures, the self-check rule,
  its once-per-game limit, and premoves.
• A CPU opponent is tested to play the King's Leap when it wins material, and never to choose it
  once spent.

Upgrade Note

Replace the previous Chess.jar and fully restart the server. The resource pack is unchanged
from v1.0.5.

Chess v1.0.6 targets Java 21 and Minecraft 1.21.11 on Spigot, Paper, or Folia.