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/assert.lua

@@ -0,0 +1,102 @@
+--- @class test.assert
+local M = {}
+
+local FORMAT_DEPTH = 100
+
+--- @param value any
+--- @return string
+local function format_value(value)
+  if type(value) == 'string' then
+    return string.format('%q', value)
+  end
+
+  local ok, inspected = pcall(vim.inspect, value, { depth = FORMAT_DEPTH })
+  if ok then
+    return inspected
+  end
+
+  return tostring(value)
+end
+
+--- @param condition boolean
+--- @param value any
+--- @param context any
+--- @param fallback string
+local function assert_value(condition, value, context, fallback)
+  if not condition then
+    local message = context ~= nil and tostring(context) or fallback
+    error(message, 0)
+  end
+  return value
+end
+
+--- @param expected any
+--- @param actual any
+--- @param comparator string
+--- @return string
+local function comparison_message(expected, actual, comparator)
+  return ('Expected values to be %s.\nExpected:\n%s\nActual:\n%s'):format(
+    comparator,
+    format_value(expected),
+    format_value(actual)
+  )
+end
+
+--- @param expected any
+--- @param actual any
+--- @param context? any
+--- @return any
+function M.eq(expected, actual, context)
+  return assert_value(
+    vim.deep_equal(expected, actual),
+    actual,
+    context,
+    comparison_message(expected, actual, 'equal')
+  )
+end
+
+--- @param expected any
+--- @param actual any
+--- @param context? any
+--- @return any
+function M.neq(expected, actual, context)
+  return assert_value(
+    not vim.deep_equal(expected, actual),
+    actual,
+    context,
+    ('Expected values to differ.\nValue:\n%s'):format(format_value(actual))
+  )
+end
+
+--- @param value any
+--- @param context? any
+--- @return any
+function M.is_true(value, context)
+  return M.eq(true, value, context)
+end
+
+--- @param value any
+--- @param context? any
+--- @return any
+function M.is_false(value, context)
+  return M.eq(false, value, context)
+end
+
+-- TODO(lewis6991): remove these aliases
+M.True = M.is_true
+M.False = M.is_false
+M.equals = M.eq
+M.Equal = M.eq
+
+return setmetatable(M, {
+  --- @param condition any
+  --- @param message? string
+  --- @param level? integer
+  __call = function(_, condition, message, level, ...)
+    if condition then
+      return condition, message, level, ...
+    end
+
+    error(message or 'assertion failed!', (type(level) == 'number' and level or 1) + 1)
+  end,
+})