pgn: escape and unescape header values

[gaoflow]

Problem

The PGN specification represents a quote inside a string as \" and a backslash as \\. Header values are string tokens, but python-chess currently exposes those representation escapes through read_game() / read_headers() and writes semantic quotes or backslashes without escaping them.

For example, a valid [Event "The \"Open\""] header is returned as The \"Open\", while assigning the semantic value The "Open" exports an invalid header with bare inner quotes.

Fix

• Decode the two PGN-defined string escapes before dispatching parsed header values. Unknown backslash sequences retain the existing forgiving import behavior.
• Escape backslashes and quotes in StringExporterMixin, covering both string and file exporters.
• Add a shared table of ordinary, quote, backslash, mixed, trailing-backslash, and bracket cases across read_game, read_headers, StringExporter, FileExporter, and export/read round trips.

Specification: https://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c7

Testing

• python test.py — 294 passed, 22 skipped (external engines/tablebases)
• python -m mypy --strict chess — clean
• python -m mypy --strict examples/**/*.py — clean
• python -m pyright chess examples/**/*.py — zero errors
• Independent exhaustive oracle: 5,461 short tag values matched PGN escaping and round-tripped

The README doctest needs a local stockfish binary; its three resulting failures reproduce unchanged on pristine origin/master.

[gaoflow]

The seven Ubuntu test jobs all stopped in .github/workflows/setup-ubuntu-latest.sh before installing this branch or running tests: apt returned 404 for libatopology2t64_1.2.11-1ubuntu0.2. Windows tests, all typing jobs, perft, and the README job passed. The README job runs apt-get update first, which is why the same Ubuntu dependency setup succeeded there. I cannot rerun failed jobs without repository admin rights.

[gaoflow]

The ubuntu job failures are environmental: the runner image's baked-in apt lists point at libatopology2t64_1.2.11-1ubuntu0.2, which the mirror no longer carries (404), and setup-ubuntu-latest.sh installs without refreshing the index. Added sudo apt-get update at the top of the script — that should unblock CI for every PR hitting image ubuntu24/20260714.240, not just this one. Happy to drop the commit if you'd rather fix it separately.