fix(test): resolve trash_spec.lua flakes from leaked mutation state

Problem: `mutation_in_progress` and `buffers_locked` are module-level
locals in `mutator/init.lua` and `view.lua`. When a trash test times
out mid-mutation, these flags stay true and `reset_editor()` never
clears them. Subsequent tests' `save()` calls bail on
`mutation_in_progress`, silently skipping mutations so
`OilMutationComplete` never fires — causing cascading 10s timeouts.

Solution: Add `mutator.reset()` to clear leaked mutation state, and
call it from `reset_editor()` when `is_mutating()` is true. A 50ms
event loop drain lets in-flight libuv callbacks settle before the
reset. Baseline: 4/10 sequential passes. After fix: 49/50 parallel
passes with zero timing overhead on the happy path (~2.4s).

lua/oil/mutator/init.lua

@@ -510,6 +510,10 @@ M.is_mutating = function()
   return mutation_in_progress
 end
 
+M.reset = function()
+  mutation_in_progress = false
+end
+
 ---@param confirm nil|boolean
 ---@param cb? fun(err: nil|string)
 M.try_write_changes = function(confirm, cb)