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).

tests/test_spell_registry_batch3.py

@@ -3,7 +3,8 @@
 Given: the registry directory with the 11 original spells plus batches 1-2 (75)
 When: load_spell_registry / load_spell is called
 Then: every batch-3 spell loads with the expected level/school/range/save,
-the registry reaches exactly 107 (11 + 96), every effect validates against the
+the registry reaches exactly 108 (11 + 96 + the SRD-conformance mage armor),
+every effect validates against the
 schema (parsing raises on any unknown field value or condition), and the
 registry covers EVERY spell name referenced by data/pcs/*.json (no missing
 spell remains).
@@ -113,9 +114,11 @@ class TestBatch3Loading:
         assert spell.base_dc == 10
         assert spell.description
 
-    def test_registry_reaches_107(self) -> None:
+    def test_registry_reaches_108(self) -> None:
         reg = load_spell_registry(_SPELLS_DIR)
-        assert len(reg) == _BATCH2_COUNT + 32 == 107
+        # +1: mage_armor.json, added by the SRD conformance pass (not a
+        # batch-3 PC-sheet spell).
+        assert len(reg) == _BATCH2_COUNT + 32 + 1 == 108
         assert set(_BATCH3) <= set(reg)
 
     def test_every_effect_validates(self) -> None: