Send polygon startboxes to the game

[burnhamrobertp]

Picks up where #574 left off, which rendered polygon startboxes in the preview but still handed the game a plain bounding-box rectangle at launch, so an offline game never got the real shape. Now the offline start script sends the start areas as modoptions, the way the game PR settled on: a selected preset goes out as the set (mapmetadata_startboxes_set, the server-owned default), and custom drag-edited boxes go out as the override (mapmetadata_startbox_override, lobby-owned) which the game prefers over the set. So an offline game enforces the real shape, custom boxes aren't silently replaced by the map's default, and online games get the set from the server.

It also brings the preview's spline in line with the game and Chobby so the curves match exactly. The old math overshot at sharp corners and could draw a small curl the game never showed.

Related PRs:

• Core game: Polygon startboxes via mapmetadata modoptions Beyond-All-Reason#7513
• Chobby: Polygon startbox infrastructure (rendering, encoder, modoption injection) BYAR-Chobby#1184
• Maps-metadata: Allow polygon and spline startboxes alongside legacy rectangles maps-metadata#615
• Rowy fork: Add polygon startbox column type p2004a/rowy#1
• Original bar-lobby PR (merged): Allow polygon start boxes #574

Assisted by Claude Code (Opus 4.8); all code reviewed and verified locally.