content: add ssik to inverse-kinematics

[jslee02]

What does this PR do?

Adds ssik to the Inverse Kinematics section.

ssik generates analytical inverse kinematics for 6R and 7R revolute robot arms — each arm becomes a self-contained Python module that returns every IK branch with FK closure tightenable to machine precision. It's the IKFast idea (per-arm solver generated at design time, pure numeric at deployment) but without IKFast's brittleness on non-Pieper 6R and non-SRS 7R geometries. Distributed on PyPI under BSD-3-Clause with a DOI.

Inclusion criteria

Must-haves (all met): robotics-relevant (IK), open source (BSD-3-Clause, on PyPI), not a duplicate (existing IK entries are symbolic/numerical — this is analytical closed-form for general geometries), meaningful description.

Scoring — 3/5 (≥3 required → Accept):

• ✅ Activity — commit within the last 2 years
• ✅ Documentation — README with install, quickstart, API, and prebuilt-arm table
• ✅ Uniqueness — analytical closed-form IK covering non-Pieper 6R / non-SRS 7R, a niche not covered by existing entries
• ❌ Popularity — 4 stars (< 50)
• ❌ Maturity — created Apr 2026 (< 6 months)

Note: README.md was regenerated. The diff also reflects the pending metadata refresh from #106, which updated data/*.yaml without regenerating README.md, so this PR brings the README back in sync (required to pass the README Freshness check).

Checklist

• I edited data/*.yaml — not README.md directly
• Entry added in alphabetical order in the correct data/*.yaml file
• python3 scripts/validate_entries.py passes
• python3 scripts/generate_readme.py run and README.md committed
• Commits are signed off (git commit --signoff)

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