Saves and progression for web games

This skill is guidance: it has no code to copy. Write the code into the game, in the game's own language, style and file layout, from the rules and examples here.

Two parts, built in this order:

1. Saves (references/saves.md): one save for the device (or one per player, with the Harmony SDK), checked on load, versioned, with Export and Import. Every game that keeps progress needs this part.
2. Progression (references/progression.md): XP, levels, currency, cosmetics, upgrades and round rewards, stored in that save. Add it only when the game wants it.

Then the screens: the results payout and the locker (references/screens.md). Last, check and hand over (below).

Harmony and saves