ssik-demo-assets: vendor headline-arm meshes for true network-free demo

[siddhss5]

Context

pip install ssik[demo] (#310) gives users viser + yourdfpy + robot_descriptions, and the demo's preload_descriptions() runs in a background thread at launch. But robot_descriptions is just a directory of Python stubs that point into ~/.cache/robot_descriptions/; the actual URDFs and meshes are lazy git cloned from upstream the first time each *_description module is touched.

Concretely on a fresh user's machine:

• pip install ssik[demo] adds maybe ~50 MB of Python deps. Fast.
• First demo launch silently triggers ~100s of MB of git clone traffic into ~/.cache/robot_descriptions/. Fails if offline / behind firewall / no git. Pins are floating (upstream HEAD).

So the demo isn't actually network-free or version-pinned.

Proposal

Publish a sibling PyPI package ssik-demo-assets that vendors meshes for a curated "headline" arm subset:

Arm	Source repo	Approx mesh size
UR5 (official)	universal-robots/Universal_Robots_ROS2_Description	~5 MB
KUKA iiwa14	IFL-CAMP/iiwa_stack	~3 MB
Unitree Z1	unitreerobotics/z1_ros2	~10 MB
Franka Panda	frankaemika/franka_ros	~15 MB
Franka Research 3	frankaemika/franka_description	~15 MB
Kinova JACO 2	personalrobotics/ada_ros2	~8 MB
FANUC CRX-10iA/L	ros-industrial/fanuc	~12 MB

Total: ~70 MB. Pin URDF + meshes at known SHAs.

ssik[demo] depends on ssik-demo-assets. The demo's local_urdf_paths is extended to look at the assets-package install location first; falls through to robot_descriptions lazy fetch otherwise.

Benefits

• pip install ssik[demo] becomes truly offline-capable for the curated subset.
• Reproducible: PyPI version pin == mesh SHA pin.
• No silent network on first launch.
• JACO 2 stops needing a local ada_ros2 checkout.

Costs

• ~70 MB sibling wheel, separately downloaded.
• New release/versioning cadence to maintain.
• License review per vendored repo (most are BSD/Apache; ada_ros2 is MIT).

Out of scope (cover later)

• Long-tail arms (Kassow, big_yam, Rizon 10, OpenArm) — add as PRs to the assets package.
• Mesh thinning / decimation if the package gets too big.

Decision needed

License check on the headline subset + agreement on the packaging cadence before this lands.

Filed as follow-up to #310 (demo polish PR).