Introduces the Xeno faction with new commanders, units, and mechanics.
- Adds audio SFX for skills: `bloodpact`, `burrow`, `carapace`, `hive_link`,
`molt-stage1`, `molt-stage2`, `swarm`, `venom-apply`, `venom-effect`.
- Adds Xeno card data to `cards.json` (commanders, assault, token units).
- Updates `skills.json`: changes `swarm` trigger from `preBattle` to `preAttack`.
- Implements combat logic in `CombatEngine.js`:
- New `_burrowPhase` to decrement burrow stacks post-battle.
- Burrowed units now take half damage from direct attacks.
- Venom kills now trigger `on_death` and `hive_link` events.
- `swarm` buff now applies in `preAttack` and reverts in `postAttack`.
- Updates `CardObject.js`: replaces burrow overlay with a stack-count badge.
- Updates `BattleScene.js`:
- Animations for `swarm` surge/revert, `hive_link` pulses, `venom` ticks,
`carapace`, `molt`, and `burrow` ticks.
- Chains `hive_link` animations immediately after death explosions.
- Adds SFX playback for all new skill effects.
