fix(output): show a car icon and "Hold for car" reason when discharge is held for a charging car

[chalfontchubby]

Written by Claude, on behalf of @chalfontchubby.

Summary

• execute.py already labels the live status "Hold for car" when car_charging_from_battery is off and a car charging slot blocks battery discharge outside a charge window (execute.py:594-629). The plan table had no equivalent - the same held SoC rendered as a plain trend arrow with no indication of why.
• Adds a hold_for_car reason code to the plan JSON, gated on the same condition execute.py/prediction.py:865 already use (set_charge_window on, car_charging_from_battery off, a car charging slot overlapping the row), so the plan's hover/tap explanation is consistent with the live status text.
• Swaps the state cell's arrow for a car icon (🚗) in that case, the same way the existing sun glyph signals a PV-driven rise - so the hold is visible at a glance, not just on hover.
• Display only. The underlying hold behaviour (battery not discharging while the car charges) was already correct; nothing about prediction or execution changes.

Fixes #5146, found while investigating #5122.

Test plan

• New test cases in test_plan_why_reason.py: a car-held Demand slot reads as hold_for_car and shows the car icon; the same slot with car_charging_from_battery on stays plain demand_steady (negative case)
• ./run_all --test plan_why_reason passes
• ./run_all --quick passes (all tests, 4 slow skipped)
• Manually replayed the reporter's own attached debug.yaml (predbat_debug_20260917-210000.yaml) - all four slots matching his car charging schedule now show the icon; plan SoC/cost numbers unchanged