Replace the busted-based Lua test runner with a repo-local harness.
The new harness runs spec files directly under `nvim -ll`, ships its own
reporter and lightweight `luassert` shim, and keeps the helper/preload
flow used by the functional and unit test suites.
Keep the file boundary model shallow and busted-like by restoring `_G`,
`package.loaded`, `package.preload`, `arg`, and the process environment
between files, without carrying extra reset APIs or custom assertion
machinery.
Update the build and test entrypoints to use the new runner, add
black-box coverage for the harness itself, and drop the bundled
busted/luacheck dependency path.
AI-assisted: Codex
Problem: Transpiled Lua code from vim9script is not amenable to static
analysis, requiring manual cleanup or ignoring parts of our codebase.
Solution: Revert to pre-rewrite version of (low-impact) legacy plugins
and remove the vim9jit shim.
Problem: No type information for `vim.uv`.
Solution: Vendor https://github.com/LuaCATS/luv (which is what
luals bundles). This will allow other tooling to work out-of-the-box and
make these files available to users and plugins without the need for
`lazydev.nvim` etc.
