Add stat modifiers and attack cost overrides to CardInstance
Support card effects that modify Pokemon stats:
- hp_modifier: Additive HP change (e.g., +20 from Giant Cape tool)
- retreat_cost_modifier: Additive retreat cost change (e.g., -99 for Float Stone)
- damage_modifier: Additive damage change for attacks
- attack_cost_overrides: dict[int, list[EnergyType]] for per-attack cost changes
Added helper methods:
- effective_hp(base_hp) - returns max(1, base_hp + hp_modifier)
- effective_retreat_cost(base_cost) - returns max(0, base_cost + modifier)
- effective_attack_cost(attack_index, base_cost) - returns override or base
Updated is_knocked_out() and remaining_hp() to use effective HP.
Also updated PROJECT_PLAN.json:
- Marked HIGH-003, TEST-006, HIGH-004 as completed (Week 2 complete)
- Updated test counts and notes to reflect stat modifier coverage
289 tests passing.
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