Module versions 

Frozen save schemas — one module per shipped version.

Each vN module is a verbatim copy of the persisted fields as they existed at the time that version shipped. Once a vN module is on main it MUST NOT be edited (except to fix a migration bug). Add vN+1.rs with a From<vN>-style conversion instead.

Modules

v1

Save schema V1 — the pre-versioned shape that shipped on main.

v2

Save schema V2 — adds the version field, per-fingerer modifiers, and Green-Coin spawn pity counter.

v3

Save schema V3 — drops goldens_since_green_coin.

v4

Save schema V4 — drops the old hardcoded upgrades_earned set entirely and introduces the infinite procedural upgrade tree.