fix: use try/finally for spellDefense cleanup instead of delete

delete on game.lethalFantasy.spellDefense was breaking roll flow
on Foundry's proxied game object.  Use try/finally with
assignment to false instead, which is safe on any object type.

Initialize saveSpell local var from one-shot flag so D30
chart lookup correctly uses arcane_spell_defense for spell saves
without requiring user to click the pre-checked checkbox.

module/documents/roll.mjs

@@ -127,6 +127,7 @@ export default class LethalFantasyRoll extends Roll {
    * @returns {Promise<Object|null>} The roll result or null if the dialog was cancelled.
    */
   static async prompt(options = {}) {
+    try {
     let dice = "1D20"
     let maxValue = 20
     let baseFormula = "1D20"
@@ -140,8 +141,6 @@ export default class LethalFantasyRoll extends Roll {
     let pointBlank = false
     let letItFly = false
     let saveSpell = game.lethalFantasy?.spellDefense ?? false
-    // Consume one-shot flag so it doesn't leak to subsequent non-spell saves
-    if (game.lethalFantasy) delete game.lethalFantasy.spellDefense
     let beyondSkill = false
     let hasStaticModifier = false
     let hasExplode = true
@@ -681,6 +680,10 @@ export default class LethalFantasyRoll extends Roll {
     if (Hooks.call("fvtt-lethal-fantasy.Roll", options, rollData, rollBase) === false) return
 
     return rollBase
+    } finally {
+      // Clear one-shot flag so it doesn't leak to subsequent non-spell saves
+      if (game.lethalFantasy) game.lethalFantasy.spellDefense = false
+    }
   }
 
   /* ***********************************************************/