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
- Move all generator Lua scripts to the `src/gen/`
- Add a `.luarc.json` to `src/gen/`
- Add a `preload.lua` to `src/gen/`
- Add `src` to `package.path` so it aligns with `.luarc.json'
- Fix all `require` statements in `src/gen/` so they are consistent:
- `require('scripts.foo')` -> `require('gen.foo')`
- `require('src.nvim.options')` -> `require('nvim.options')`
- `require('api.dispatch_deprecated')` -> `require('nvim.api.dispatch_deprecated')`
