Branch: Akunur v3.1 prototype
Release: 3.1.0-alpha.2
Purpose: Reserve lifecycle on the multi-player core foundation
Parent: 3.1.0-alpha.1
Status: Experimental rules build / two-player browser enabled

Scope:
- core supports 2 or 3 active players;
- third player is N = Natural/Wood;
- normal captured ordinary stones still echo radially;
- pieces displaced by a radial Echo enter reserve;
- captured Echo Pieces lose Echo state and enter reserve as ordinary pieces;
- one reserve piece may replace a normal move by returning to an empty original starting cell;
- browser and existing agents support the new reserve-return action in two-player games;
- three-player browser/AI remains deferred.
