perf python: Port stat-cpi to perf module

Port stat-cpi.py from the legacy embedded scripting framework to a
standalone Python script in tools/perf/python/ to calculate Cycles Per
Instruction (CPI) per interval per CPU or thread.

Improvements compared to the legacy script:
- Support both perf.data file mode (via perf.session stat callbacks)
  and live counter collection mode (using perf.parse_events,
  evlist.open, and evsel.read across intervals), with automatic fallback
  to user-space (:u) and self-process monitoring when perf_event_paranoid
  restricts system-wide events (EACCES).
- Compute per-interval counter deltas (val, ena, run) keyed by raw event
  name so cumulative PERF_RECORD_STAT snapshots and hybrid PMU events
  (e.g. cpu_core/cycles/, cpu_atom/cycles/) are accumulated accurately,
  and scale counts by time_enabled / time_running when multiplexed.
- Replace hard-coded CPU ([0, 1]) and thread ([0]) arrays with dynamic
  CPU and thread discovery so arbitrary system topologies work
  automatically.
- Add CLI option handling (-i, -I, -p) via argparse and type annotations
  passing mypy and pylint.

Add a shell test (test_stat_cpi_python.sh) to verify the standalone
script.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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