Experimental physics source adaptation
Reference: https://github.com/NarukamiTO/client/tree/main/game/src
Game module documented as 2019-07-19. See ../LICENSE and ../NOTICE.txt.
PhysicsScene is retained from the reference; Havyra is adapted TrackedChassis and Tezy is adapted SuspensionRay.
Identifiers retain our client ABI. Existing spawn movement lock is preserved. Keyboard steering defaults to full 7/7. Legacy turn acceleration supplies the stabilization default until a separately verified original parameter set is available.
Hull balance, map gravity/collision geometry, and server movement validation are not replaced. No runtime toggle.
