Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

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