Add ``game_id`` argument to PPN Viewer

[trevorld]

• Hide if there is only one game
• Select which game in PPN to show
• Default to 1

[trevorld]

❯ How do different game notation formats record a game id (if they do at all) so it is
easy to pull up a specific game in a collection of games?

● The approaches vary quite a bit across formats:

PGN has no dedicated ID field. Instead it relies on the "Seven Tag Roster" — the
combination of Event, Site, Date, Round, White, and Black tags — as a quasi-unique key.
In practice chess databases just use a sequential integer row number internally and
build indexes over those tags. Some non-standard extensions add [GameID "..."] or [ECO
"..."] for opening classification.

SGF similarly has no unique ID. Games are identified by filename, or by the combination
of GN[] (game name), DT[] (date), PB[]/PW[] (players). Online Go servers often embed a
server-specific game number in the filename rather than in the format itself.

PBN (Bridge) uses the [Board "N"] tag prominently because board number is meaningful in
bridge tournaments. Still no globally unique ID — identification is by
Event+Round+Board.

PDN follows PGN conventions closely; same situation.

PTN (Tak): PlayTak.com embeds the game number in metadata when exporting, but it's
site-specific.

The common pattern across all of them: none define a standard unique game ID field.
Identification is either by filename, by a composite of metadata tags (players, event,
date, round), or by database row number assigned externally.

If PPN wanted to do better, a GameID field accepting a UUID or a short human-readable
slug would be straightforward to add — YAML handles it naturally and it wouldn't
conflict with anything. A UUID has the advantage of being collision-resistant for
distributed recording; a slug like "alice-vs-bob-2026-04-25-g1" is more human-friendly
for small collections.