feat: configurable fps smoothing (SampleCount) + refresh (DisplayUpdateMs)

[si978]

Task ID: TASK-2025-12-16-0006

Summary:

• overlay.ini adds:
  • SampleCount: smoothing window length (frames)
  • DisplayUpdateMs: display refresh period (ms)
• Wires these into FpsCounter and updates default overlay.ini template + docs.

Example:

[Overlay]
SampleCount=60
DisplayUpdateMs=80

Validation:

• python .ai/tools/memctl.py validate --commit HEAD
• python .ai/tools/memctl.py check-stale --commit HEAD
• python .ai/tools/memctl.py build-pack --commit HEAD --task-id TASK-2025-12-16-0006 --out context_pack.json
• python .ai/tools/memctl.py validate-pack --pack context_pack.json --task-id TASK-2025-12-16-0006
• python -m unittest discover -s .ai/tests -p "test_*.py"