refactor: split util/ vs features/ + modular opt-in setupX() config

Package restructure to prepare future modularization. Three layers now:

1. fr.luc.crcore.util.* (always active)
   - common, command framework, database, message, broadcast, gui,
     placeholder

2. fr.luc.crcore.features.* (opt-in)
   - team (entities, services, repos, events, exceptions, config + GUI,
     command/Team*SubCommand)
   - player (profile, ranking, services, repos, events, exceptions)

3. fr.luc.crcore.builtin.*
   - CoreCommand + CoreReloadSubCommand (top-level routing — not a
     feature, not an util)

FQN moves (game plugins importing these need their imports updated):
- fr.luc.crcore.common.*          → fr.luc.crcore.util.common.*
- fr.luc.crcore.command.*         → fr.luc.crcore.util.command.*
- fr.luc.crcore.command.builtin.team.*
                                  → fr.luc.crcore.features.team.command.*
- fr.luc.crcore.command.builtin.CoreCommand / CoreReloadSubCommand
                                  → fr.luc.crcore.builtin.*
- fr.luc.crcore.database.*        → fr.luc.crcore.util.database.*
- fr.luc.crcore.message.*         → fr.luc.crcore.util.message.*
- fr.luc.crcore.broadcast.*       → fr.luc.crcore.util.broadcast.*
- fr.luc.crcore.gui.*             → fr.luc.crcore.util.gui.*
- fr.luc.crcore.placeholder.*     → fr.luc.crcore.util.placeholder.*
- fr.luc.crcore.team.*            → fr.luc.crcore.features.team.*
- fr.luc.crcore.player.*          → fr.luc.crcore.features.player.*

CRCoreConfig — features opt-in via fluent setup:
- setupTeams()        enables team feature
- setupPlayers()      enables player feature
- setupPlaceholders() enables PAPI integration
- setupAll()          shortcut
By default no feature is active. Game plugin must opt-in.

CRCore.enable() now conditional:
- Util services always built (messages, broadcasts, gui listener).
- Team services + repos + config built only if setupTeams().
- Player services + repos built only if setupPlayers().
- Placeholder hook registered only if setupPlaceholders() (and PAPI
  installed at runtime).
- Getters of disabled features throw IllegalStateException with a clear
  message pointing to the missing setupX() call.
- CoreCommand only registers TeamGroupSubCommand if teamService and
  teamConfig are non-null. CoreReloadSubCommand handles teamConfig=null
  (just skips that reload). /core reload always available.

Docs:
- setup.md tree fully rewritten to reflect util/+features/+builtin
  layout (~70 lines of arborescence).
- setup.md code snippet shows the three opt-in patterns (setupAll /
  granular / with options).
- decisions.md logs both decisions (restructure + modular setup).
- All .puml diagrams auto-updated to new FQNs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/diagrams/player-class-diagram.puml

@@ -6,7 +6,7 @@ hide empty members
 
 ' === Common abstractions ===
 
-package "fr.luc.crcore.common" {
+package "fr.luc.crcore.util.common" {
 
   interface Identifiable {
     + getId(): UUID
@@ -40,7 +40,7 @@ package "fr.luc.crcore.common" {
 
 ' === Player domain ===
 
-package "fr.luc.crcore.player" {
+package "fr.luc.crcore.features.player" {
 
   class PlayerProfile {
     - scores: Map<String, Integer>