[mvanhorn]

Chess: load a PGN to review a game

The move list already copies and downloads a game as PGN. This adds the other direction: paste a PGN, or open a .pgn file, and step through it on the board.

What changes

• Load a PGN sits next to copy and download on the move list. It opens a dialog: paste the game, or choose a file, then Load.
• The first game in the dump is replayed as Pass & Play, with no clock. Player names from the PGN show on the cards. Comments, ; line comments, $1-style glyphs, and parenthetical variations are ignored. A [FEN] tag is honoured, so a game that did not start from the initial array still plays.
• The existing review controls and arrow keys step through the loaded moves. If the PGN records a result the engine cannot see (a resignation, a draw by agreement), that result is shown on the banner without opening the game-over dialog.
• A game in progress asks before it is replaced. An online match in progress asks before leaving.

Tests

• The engine now accepts a SAN string (Nf3, O-O, e8Q, e2e4) as well as {from, to}.
• parsePgn / replayPgn cover tags, glued move numbers, comments, variations, a FEN start, an illegal move, a FEN the engine will not load, and a round-trip of Scholar's mate through export then import.
• npm test: 150 passed. npm run build succeeds.

Test plan

• cd src/chess && npm ci && npm test && npm run build
• New game, play a few moves, copy PGN, New game, Load that paste, step back to the start with the arrows
• Load a .pgn file that includes {comments}, a (variation), and a result; confirm the variation is not played
• Load a PGN with a [FEN] tag that is not the starting position
• Load garbage text and confirm the dialog says why it failed, without touching the board

Not included

• Only the first game in a multi-game file is loaded.
• Variations are skipped rather than offered as a side line.
• Descriptive notation (P-K4) is not read.

Demo