#905 Clusters: link lesson plans and slides from each sub-cluster

Adds the FORRT Open Science Curriculum lesson plans to the cluster pages.

What it shows

• A "Teaching materials" strip in each sub-cluster that has a set, below the sub-cluster description and above the readings. It links the slides (PPTX, PDF), each lesson plan (DOCX, PDF) and extra materials, and names the creator, reviewers, licence (CC BY-NC-SA 4.0) and Zenodo DOI.
• A green dot on the sub-cluster header, in the sidebar, and in the cluster header ("· ● N with lesson plans").
• A "● Lesson plans" filter chip next to the existing filters. It shows only sub-clusters with sets, and it explains the dot.

Data

• data/lesson_plans.json is keyed by cluster number and the sub-cluster anchor slug. The slug formula is now in one partial (clusters/sc_slug.html), shared by the section, the sidebar and the check.
• The build fails if a key matches no sub-cluster, so renaming a sub-cluster cannot silently drop its plans.
• The file lists 8 published Zenodo records (2.5, 2.6, 3.5, 5.2, 7.3, 7.4, 7.6, 9.1). Sets 2.1 and 9.19 stay as Zenodo drafts until placeholders on their slides are fixed.

Checks

• hugo builds cleanly.
• Every file link in the data file returns HTTP 200.
• Checked visually on desktop and mobile widths.

🤖 Generated with Claude Code

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Summary by cubic

Adds the FORRT Open Science Curriculum lesson plans and slides to cluster pages, linking each published Zenodo record from the sub-cluster it belongs to.

• A "Teaching materials" strip below each sub-cluster description links slides (PPTX, PDF), lesson plans (DOCX, PDF), and extra materials, naming the creator, reviewers, the CC BY-NC-SA 4.0 licence, and the Zenodo DOI; it names the set when it sits under a differently named sub-cluster. Chips wrap on narrow screens.
• Sub-clusters with sets show a green dot on their header, in the sidebar, and in the cluster stats ("· ● N with lesson plans").
• An "Only sub-clusters with lesson plans" checkbox combines with the existing search and tag filters; sub-clusters and clusters without sets are hidden, with a message for clusters that have none. When on, sub-clusters with a set stay open even when no other filter matches their cards, and the empty-cluster message is announced to screen readers.
• data/lesson_plans.json is keyed by cluster number and sub-cluster anchor slug; the build fails if a key matches no sub-cluster, so renaming a sub-cluster cannot silently drop its materials.
• The slug formula lives in one shared partial (clusters/sc_slug.html) used by the section, sidebar, and data validation.
• Covers 8 published Zenodo records (2.5, 2.6, 3.5, 5.2, 7.3, 7.4, 7.6, 9.1); sets 2.1 and 9.19 remain drafts until placeholders on their slides are fixed.