car tests: cache CANDefine values and speed KF gain

[javery556]

Refs commaai/openpilot#32536

Summary

• cache CANDefine values per DBC so car state init does not rebuild the same enum maps for every Hypothesis example
• precompute the constant speed-KF gain used by CarStateBase instead of solving it again on each initialization

Validation

• Dongle ID: N/A (no car changes)
• Route: N/A (no car changes)

Test plan

• local xdist run of pytest selfdrive/car/tests/test_car_interfaces.py --durations=20 -q
  • before: 241 passed in 7.77s
  • after: 241 passed in 6.42s
• local single-process run of pytest selfdrive/car/tests/test_car_interfaces.py -n0 --durations=20 -q
  • after: 241 passed in 46.94s
  • slowest test: 0.94s
  • average per test: about 0.195s
• local spot check of historically slow cars with pytest selfdrive/car/tests/test_car_interfaces.py -n0 -k 'FORD_FOCUS_MK4 or MAZDA_CX9_2021 or SKODA_FABIA_MK4' --durations=10 -q
  • FORD_FOCUS_MK4: 0.19s
  • MAZDA_CX9_2021: 0.15s
  • SKODA_FABIA_MK4: 0.17s

[github-actions]

Car behavior report

Replays driving segments through this PR and compares the behavior to master.
Please review any changes carefully to ensure they are expected.

✅ No changes detected

[github-actions]

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