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).

data/spells/mage_armor.json

@@ -0,0 +1,20 @@
+{
+  "name": "Mage Armor",
+  "level": 1,
+  "school": "conjuration",
+  "range": "touch",
+  "save": "none",
+  "base_dc": 10,
+  "cast_time": "standard",
+  "spell_resistance": false,
+  "effects": [
+    {
+      "type": "buff",
+      "modifiers": [
+        { "target": "ac", "value": 4, "bonus_type": "armor" }
+      ],
+      "duration_rounds": 600
+    }
+  ],
+  "description": "An invisible but tangible field of force surrounds the subject, providing a +4 armor bonus to AC (SRD mage armor: school conjuration (creation) [force], range touch, duration 1 hour/level (modeled as a fixed 600 rounds), spell resistance: no; Will negates (harmless) is not modeled - the engine only self/target-buffs willing recipients). Known simplification: the engine bakes worn armor into the AC total of the JSON and cannot see it, so this armor bonus stacks additively with that baked-in armor value instead of replacing it; two mage armors still do not stack with each other (armor is a non-stacking bonus type)."
+}