InventoryGrid class

A data-driven grid over an InventorySnapshot — like QuestViewModel/ LeaderboardEntry, this widget holds no game logic or catalog of its own; itemBuilder is the caller's own renderer (item icon, rarity frame, stack count, equipped badge — this package doesn't ship a concrete item catalog, see ItemDefinition's doc), and a tap never mutates anything here — the caller's own tap handler is expected to call InventoryService.consume/setEquipped/moveSlot itself.

Cells beyond snapshot.slots.length render via emptyBuilder (falls back to a plain dim placeholder) up to unlockedCapacity (default: snapshot.capacity, meaning no locked cells at all), then lockedBuilder for the rest — e.g. capacity the player hasn't unlocked yet with an IAP/level-up. InventorySnapshot itself has no notion of "locked", so this is purely a presentation concept the caller opts into via unlockedCapacity.

Stable identity across reorder/update: each filled cell is keyed by InventorySlot.slotId (never by its list index) via KeyedSubtree — the exact same reasoning as InventorySlot.slotId's own doc comment ("survives a grant/consume elsewhere in the inventory changing the list's indices"). Without this, Flutter would reuse a tile's State/subtree by POSITION when the list reorders, silently handing item A's rendered state (an in-progress local animation, say) to wherever item B now sits.

Lazy: built on GridView.builder — only visible cells are ever built/laid out, regardless of how large unlockedCapacity is.