HP formula stored on participant, loads in edit modal

hpFormula field persisted on participant doc (makeParticipant + builder).
Add handler stores formula. Edit modal loads participant.hpFormula, reroll
button sets maxHp field (no save until save). Edit submit persists formula.
Formula only for monster/npc types.

TODO.md

@@ -14,6 +14,11 @@ not sure good way to do this
 lots of updates
 
 
+## monsters per campaign and npcs
+## or/and ....copy from encoubnter?
+
+
+
 
 ### TEST GAP: current branch changes need focused coverage
 - Storage `where()` contract: firebase + server adapters should honor `[where('encounterPath','==',x), orderBy('ts','desc'), limit(n)]`.