fix: cancel active process on contest switch (#316)

## Problem

Switching contests while a run, interactive session, or stress test was
active left orphaned callbacks and terminal jobs alive. Running `:CP
run`
twice also let the first run's stale output overwrite the buffer.

## Solution

Replace the `io_view_running` bool in `views.lua` with a generation
counter so concurrent `run_io_view` calls self-cancel via stale-gen
checks in async callbacks. Add `cancel_io_view`, `cancel_interactive`,
and `stress.cancel` for forceful teardown, and call them in
`setup_contest` whenever `is_new_contest` is true.

lua/cp/stress.lua

@@ -232,4 +232,18 @@ function M.toggle(generator_cmd, brute_cmd)
   end)
 end
 
+function M.cancel()
+  if state.stress_buf and vim.api.nvim_buf_is_valid(state.stress_buf) then
+    local job = vim.b[state.stress_buf].terminal_job_id
+    if job then
+      vim.fn.jobstop(job)
+    end
+  end
+  if state.saved_stress_session then
+    vim.fn.delete(state.saved_stress_session)
+    state.saved_stress_session = nil
+  end
+  state.set_active_panel(nil)
+end
+
 return M