Link NaturalProductMech records to its own pages, not GitHub

[realmarcin]

Closes #149, closes #229.

NaturalProductMech commits one page per record, pages/<class>/<slug>.html, mirroring data/natural_products/<class>/<slug>.yaml. Its branch build (build_type legacy, main, /) serves them. The census record lists in the heatmap cells and overlap panels now open those pages instead of the YAML on GitHub.

Changes

• Link base: in scripts/fleet/build_subsets.py, SITE_BASE["NaturalProductMech"] is now https://culturebotai.github.io/NaturalProductMech/pages/, and slug_for emits <class>/<slug>.html.
• Panel wording: in _fleet/fleet_fragment.html, the cell panel says "source file on GitHub" only for CultureMech and MediaIngredientMech.
• Docs: _fleet/README.md says which Mechs link where.
• Rebuilt data: assets/fleet/ was rebuilt with build_subsets.py at the Refresh the X-Mech page from the Mechs as they are now, and add the skill that does it #120 pins, from the same snapshot, followed by build_data.py and assemble_page.py.

Evidence

• What changed: exactly the 15 NaturalProductMech cell and edge indexes. A structural comparison of old and new JSON finds no differences other than the link base and the .yaml → .html slug suffix. subsets_summary.json and fleet_data.json are byte-identical.
• Every link resolves: all 3,115 records at pin 36662aa have a committed page (git ls-tree), and all 2,576 distinct records the indexes link are among them. Four sampled live URLs return 200.
• Checks: python3 -m unittest discover -s tests passes, and assemble_page.py --check reports current. In mechs.md, only the panel-wording line changes.

Left out

CultureMech keeps its GitHub links. Its pages/media/ pages come and go because its Pages source (branch builds) and its Actions deployment override each other (#175). That half of the original issue is now #223, blocked on CultureMech#489.

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