refactor(org): add project-prefixed CUSTOM_ID tags to all headings

refactor(org): add project-prefixed CUSTOM_ID tags to all headings

Replace the meaningless ":CUSTOM_ID: general" placeholder that appeared
on every project's * General heading with a unique kebab-case slug
prefixed by the project name. Add CUSTOM_ID tags to all other headings
that previously had none.

This makes HTML anchors readable (e.g. #nano-vna-case-dimensions
instead of #general) and brings the org content in line with the
sixth-3d documentation site convention.

All 112 CUSTOM_IDs are globally unique; the one collision (inner
"** Optics" vs top-level "* Optics" in index.org) was resolved by
renaming the inner one to "cases-optics".