fix(isaacgym): default projectiles=1 + match hidden-projectile layout…

[jyf588]

… across backends

Fixes #222.

• Add ProjectileConfig.hide_spacing (4 m default) + hidden_z_for_index() helper so hidden cubes get unique z slots; use it at both spawn time (all 4 backends) and runtime hide paths (_update_projectiles / _hide_projectiles_for_envs).
• IsaacGym: override SimulatorConfig.projectile default_factory to ProjectileConfig(num_projectiles=1). Each cube is a full dynamic rigid body that allocates PhysX contact patches against the wildcard-collidable add_triangle_mesh terrain; at 4096 envs × 5 cubes this exceeded the ~80k maxRigidPatchCount budget (not configurable from gymapi.PhysXParams). Other backends keep the 5-cube default. Override via --overrides simulator.projectile.num_projectiles=N to opt back into more cubes.
• IsaacGym / IsaacLab / Newton: in _set_projectile_root_states, rewrite hidden cubes' X/Y by env_id (z <= hide_z is the hidden marker, throws skip this) so post-init projectile layout is consistent across backends.
• base_simulator._throw_projectile: early-return when num_projectiles==0 so backends that opt out can't crash on the J key.