feat(rules): SRD Mage Armor spell + non-stacking armor bonus type

SRD spells/mage-armor.md: 'An invisible but tangible field of force
surrounds the subject of a mage armor spell, providing a +4 armor bonus
to AC' - 'School conjuration (creation) [force]', level 1, 'Casting Time
1 standard action', 'Range touch', 'Duration 1 hour/level (D)',
'Spell Resistance no'.

- data/spells/mage_armor.json: buff +4 armor to AC, 600 rounds (1
  hour/level modeled fixed, the shield.json convention). Description
  documents the known simplification: the engine bakes worn armor into
  the AC total and cannot see it, so the armor bonus stacks additively
  with that baked value (two mage armors still do not stack).
- effects.py: 'armor' added to BonusType; absent from _STACKING_TYPES it
  behaves as a keep-the-highest type (verified by resolve_modifiers
  tests).
- Spell-count pins 107 -> 108 (test_runner_spells, batch3, test_spells)
  and the README references.
- Golden transcript untouched (buff branch only fires when no offensive
  spell is selected; the lich still opens with Magic Missile).

src/pf1e_simulator/effects.py

@@ -3,8 +3,8 @@
 PF1e bonus types determine whether multiple bonuses to the same stat stack:
 - Stacking types (dodge, racial, trait, untyped/"") all sum together.
 - Non-stacking types (morale, sacred, profane, competence, insight, luck,
-  enhancement, deflection, natural, shield) keep only the highest value per
-  type.
+  enhancement, deflection, natural, shield, armor) keep only the highest
+  value per type.
 - Penalties follow the same rules: same-type penalties don't stack (the
   least severe — i.e. the max value — is kept).
 
@@ -38,6 +38,7 @@ BonusType = Literal[
     "deflection",     # no stack
     "natural",        # no stack
     "shield",         # no stack
+    "armor",          # no stack (SRD mage armor: "+4 armor bonus to AC")
     "circumstance",   # no stack (simulator can't distinguish circumstances)
 ]