Populate campaign players array in combat replay and scenario

Both combat.js replay and Combat.scenario.test.js built character participants
inline via buildCharacterParticipant without persisting to campaign doc. Real
app stores chars in campaign doc players array; encounter participants built
from that. Replay/scenario diverged from real flow — campaign had zero chars.

Fix:
- replay.js: roster chars now have id, persisted to campaign doc players array
  at setup_campaign step
- Combat.scenario.test.js: addCharacterViaUI writes char to campaign doc
  players array (mirrors app), CAMPAIGN_PATH const added

TODO.md

@@ -7,6 +7,7 @@ Backlog of bugs + long-term items. Milestones live in REWORK_PLAN.md.
 
 
 ### dm list - keep active particpant in view (scroll)
+not sure good way to do this
 
 ### combat.js doesnt seem to actually exercise the add characters.  there are no characters in the campaign.   only in the encounter..
 
@@ -99,6 +100,9 @@ Backlog of bugs + long-term items. Milestones live in REWORK_PLAN.md.
 - Migration: keep old log entries readable; new format for new writes.
 
 
+### quality of life fix: 2. UI says "Campaign Characters", field is players --- naming mismatch (separate concern, flag for later)
+
+
 ## FEAT - parallel campaigns
 
 ## FEAT - multi user