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>

src/pf1e_simulator/effects.py

@@ -3,7 +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) keep only the highest value per type.
+  enhancement, deflection, natural, shield) 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).
 
@@ -36,6 +37,7 @@ BonusType = Literal[
     "enhancement",    # no stack
     "deflection",     # no stack
     "natural",        # no stack
+    "shield",         # no stack
     "circumstance",   # no stack (simulator can't distinguish circumstances)
 ]