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
Some benchmarks:
MAIN_CDEFS + NO_TRACE: 3.81s user 1.65s system 33% cpu 16.140 total
MAIN_CDEFS: 73.61s user 10.98s system 154% cpu 54.690 total
NO_TRACE: 18.49s user 4.30s system 73% cpu 30.804 total
(default): 77.11s user 14.74s system 126% cpu 1:12.79 total