* fix(compiler): guard active entry before clearing in process callback
Problem: when M.compile() is called while a previous process is still
running, the old process's vim.schedule_wrap callback unconditionally
sets active[bufnr] = nil, wiping the new process from the tracking
table. status() incorrectly returns idle and stop() becomes a no-op
against the still-running process.
Solution: capture obj as an upvalue in each callback and only clear
active[bufnr] if it still points to the same process object.
* fix(compiler): hoist obj declaration before vim.system closure
Problem: lua-language-server flagged obj as an undefined global in
both vim.schedule_wrap callbacks because local obj = vim.system(...)
puts the variable out of scope inside the closure at declaration time.
At runtime the guard active[bufnr].obj == obj evaluated obj as nil,
so the clear was always skipped and the process remained tracked
indefinitely.
Solution: split into local obj / obj = vim.system(...) so the
upvalue is in scope when the closure is defined.
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