feat(rules): conform bulls strength, bears endurance and shield to the SRD

Bull's Strength now grants the SRD +4 enhancement bonus to Strength as
+2 melee attack, +2 melee damage and +2 CMB (three modifiers instead of
the previous +1/+1 approximation). Shield now uses a real shield bonus
type (+4 AC) instead of approximating it as deflection: the engine's
BonusType literal and stacking docs gain the shield type, which stays
out of the CMD bonus list (SRD Combat Maneuver Defense). Bear's
Endurance keeps its +2 Fortitude / +2 hp per HD and its description now
cites the SRD and flags the hp modifier as informational. Golden
transcripts updated: the buff log reports 3 mods and Tammara's pistol
attacks gain the extra +1 enhancement (d20=10+9-6=15).

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

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

tests/test_spells.py

@@ -193,7 +193,9 @@ class TestSpellLoading:
         spell = load_spell(_SPELLS_DIR / "bulls_strength.json")
         eff = spell.effects[0]
         assert isinstance(eff, BuffEffect)
-        assert len(eff.modifiers) == 2
+        # SRD bull's strength: +4 enhancement Str -> +2 melee attack, +2 melee
+        # damage, +2 CMB (three modifiers).
+        assert len(eff.modifiers) == 3
 
     def test_summon_monster_casts_for_one_round(self) -> None:
         # PF1e: Summon Monster I/II have a 1-round casting time.
@@ -616,10 +618,13 @@ class TestBullsStrength:
         buff_mods = [m for b in t.active_buffs for m in b.modifiers]
         atk_mods = [e for e in buff_mods if e.target == "attack"]
         dmg_mods = [e for e in buff_mods if e.target == "damage"]
+        cmb_mods = [e for e in buff_mods if e.target == "cmb"]
         assert len(atk_mods) == 1
         assert len(dmg_mods) == 1
-        assert atk_mods[0].value == 1
-        assert dmg_mods[0].value == 1
+        assert len(cmb_mods) == 1
+        assert atk_mods[0].value == 2
+        assert dmg_mods[0].value == 2
+        assert cmb_mods[0].value == 2
 
     def test_transcript_shows_buff(self) -> None:
         caster = _make_caster()