feat(rules): add ongoing spell damage

Acid Arrow (SRD spells/acid-arrow.md) deals 2d4 acid, then 2d4 again at the start of each of the target's turns for 1 round + 1 per three caster levels (max 6). DamageEffect gains a RecurringDamageSpec (JSON 'recurring'); a landed hit attaches a RecurringDamage source to CombatantState.recurring_damage, ticked in _turn_gate (after bleed), reduced by energy resistance/immunity but never DR (spell damage), cleared at expiry, and exposed in the observation snapshot. Extracted _resolve_one_projectile to keep _apply_damage_effect under the complexity ceiling. Updated acid_arrow.json, added tests/test_recurring_damage.py, and the README.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

data/spells/acid_arrow.json

@@ -12,8 +12,15 @@
       "type": "damage",
       "formula": "2d4",
       "types": ["acid"],
-      "half_on_save": false
+      "half_on_save": false,
+      "recurring": {
+        "formula": "2d4",
+        "types": ["acid"],
+        "base_rounds": 1,
+        "per_levels": 3,
+        "max": 6
+      }
     }
   ],
-  "description": "SRD spells/acid-arrow.md: ranged touch attack dealing 2d4 acid damage, plus another 2d4 per round for 1 extra round per three caster levels (max 6 additional rounds). The engine has no damage-over-time subsystem, so only the initial 2d4 is modeled - the ongoing acid is documented as a limitation. No save."
+  "description": "SRD spells/acid-arrow.md: ranged touch attack dealing 2d4 acid damage, plus another 2d4 acid damage at the start of each of the target's turns for 1 round + 1 round per three caster levels (maximum 6 additional rounds). Recurring damage is reduced by energy resistance/immunity, not by DR. No save."
 }