test: replace busted with local harness

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

test/functional/lua/buffer_updates_spec.lua

@@ -8,7 +8,6 @@ local api = n.api
 local fn = n.fn
 local clear = n.clear
 local eq = t.eq
-local fail = t.fail
 local exec_lua = n.exec_lua
 local feed = n.feed
 local expect_events = t.expect_events
@@ -507,7 +506,7 @@ describe('lua: nvim_buf_attach on_bytes', function()
       for _, event in ipairs(events) do
         for _, elem in ipairs(event) do
           if type(elem) == 'number' and elem < 0 then
-            fail(string.format('Received event has negative values'))
+            error('Received event has negative values')
           end
         end