Problem: no built-in support for compiling mermaid diagrams via `mmdc`.
Solution: add a `mermaid` preset that compiles `.mmd` files to SVG and
parses `Parse error on line N` diagnostics from stderr. Add
`mermaid-cli` to the nix dev shell.
Problem: PlantUML (`.puml`) diagrams have no built-in preview support,
and Neovim lacks filetype detection for PlantUML files.
Solution: Add a `plantuml` preset that compiles to SVG via `plantuml
-tsvg`, with an error parser for `Error line N` diagnostics. Register
`.puml` and `.pu` extensions via `vim.filetype.add` when the preset is
configured. Add `plantuml` to the nix dev shell.
Problem: reload_spec.lua called io.open() without nil checks, causing
need-check-nil warnings. Adding ${3rd}/busted and ${3rd}/luassert to
workspace.library caused lua-language-server 3.7.4 to run diagnostics
on its own bundled meta files, surfacing pre-existing cast-local-type
bugs in luassert's annotations that are not ours to fix.
Solution: use assert(io.open(...)) in reload_spec.lua to satisfy the
nil check. Remove busted/luassert library paths from .luarc.json since
they only benefit spec/ which is not type-checked in CI. Narrow the
lua-language-server check in scripts/ci.sh to lua/ to match CI.
