build: migrate test framework from plenary to busted

Problem: plenary.nvim is deprecated. The test suite depends on
plenary's async test runner and coroutine-based utilities, tying the
project to an unmaintained dependency. CI also tests against Neovim
0.8-0.11, which are no longer relevant.

Solution: replace plenary with busted + nlua (nvim -l). Convert all
async test patterns (a.wrap, a.util.sleep, a.util.scheduler) to
synchronous equivalents using vim.wait. Rename tests/ to spec/ to
follow busted convention. Replace the CI test matrix with
nvim-busted-action targeting stable/nightly only. Add .busted config,
luarocks test_dependencies, and update the nix devshell.

spec/minimal_init.lua

@@ -0,0 +1,8 @@
+vim.cmd([[set runtimepath=$VIMRUNTIME]])
+vim.opt.runtimepath:append('.')
+vim.opt.packpath = {}
+vim.o.swapfile = false
+vim.fn.mkdir(vim.fn.stdpath('cache'), 'p')
+vim.fn.mkdir(vim.fn.stdpath('data'), 'p')
+vim.fn.mkdir(vim.fn.stdpath('state'), 'p')
+require('spec.test_util').reset_editor()