feat: inventory system, adventure scene engine, and PC generator fixes

- Inventory: items linked to PCs with name, qty, ENC, category; effective
  FP = STR+CON minus total ENC carried; CRUD API + inline UI on Characters tab
- Adventure scene engine: new tab with procedural CYOA choices (Fight, Sneak,
  Talk, Investigate, Flee) resolved via skill checks; scenes generated from
  any rollable table with automatic L2 cascade; scene state persisted in DB
- Fix total HP formula: was CON+SIZ, now correctly ceil((CON+SIZ)/2) per RQ3
- Fix hit location HPs: were computed from wrong total HP, now correct
- Add fatigue points (STR+CON) to derived stats
- Add all seven skill category modifiers computed from characteristics
  (primary/secondary/negative influences per rulebook), shown in generator UI
- Add base skill computation (computeBaseSkills) for use in scene engine
- Add RQ3 Players Book to docs/ as reference

public/index.html

@@ -24,6 +24,8 @@
       <button class="tab-btn" data-view="npcs">NPCs</button>
       <button class="tab-btn" data-view="enemies">Enemies</button>
       <button class="tab-btn" data-view="combat">Combat</button>
+      <button class="tab-btn" data-view="characters">Characters</button>
+      <button class="tab-btn" data-view="adventure">Adventure</button>
     </nav>
     <div id="sidebar-content"></div>
   </aside>