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.

Makefile

@@ -11,13 +11,13 @@ all: lint test
 ## test: run tests
 .PHONY: test
 test:
-	./run_tests.sh
+	luarocks test --local
 
 ## lint: run selene and stylua
 .PHONY: lint
 lint:
 	selene --display-style quiet .
-	stylua --check lua tests
+	stylua --check lua spec
 
 ## profile: use LuaJIT profiler to profile the plugin
 .PHONY: profile
@@ -41,4 +41,4 @@ scripts/benchmark.nvim:
 ## clean: reset the repository to a clean state
 .PHONY: clean
 clean:
-	rm -rf .testenv perf/tmp profile.json
+	rm -rf perf/tmp profile.json