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>

data/spells/bears_endurance.json

@@ -16,5 +16,5 @@
       "duration_rounds": 60
     }
   ],
-  "description": "The subject gains a +4 enhancement bonus to Constitution, granting +2 Fortitude and +2 hit points (approximated as +2/+2). Fortitude negates (harmless). Duration 10 min/level approximated as 60 rounds."
+  "description": "The subject gains a +4 enhancement bonus to Constitution, granting +2 to Fortitude saves and +2 hit points per Hit Die (SRD bear's endurance; the hp modifier is informational — the engine has no per-HD hp pool). Will negates (harmless). Duration 10 min/level approximated as 60 rounds."
 }