Add per-island inventory switching (#40)

[tastybento]

Summary

• Adds per-island inventory switching so players with multiple concurrent islands get separate inventories per island, preventing cross-island resource cheating
• Controlled by a new options.islands config section with granular sub-options for each aspect (inventory, health, food, advancements, gamemode, experience, ender-chest, statistics)
• Inventories switch when entering an owned island's protection zone, teleporting between owned islands, or respawning on a different owned island after death
• Island inventories stay consistent across Overworld/Nether/End dimensions; generic nether/end preserves the current island's inventory
• No behavior change for games without concurrent island ownership or players with a single island
• Backward compatible: existing world-only data migrates to the first island that loads it

Config

options:
  islands:
    active: true
    inventory: true
    health: false
    food: false
    advancements: false
    gamemode: false
    experience: false
    ender-chest: true
    statistics: false

Test plan

• All 75 unit tests pass (18 new tests added)
• Player with 1 island: world switching works as before
• Player with 2+ islands: teleporting between them switches inventory
• Walking across island protection zone border triggers inventory switch
• Die on island A, respawn on island B: island B inventory loaded
• Generic nether: keep current island inventory, switch on return to different island
• Mixed config (e.g., inventory per-island, health shared): only configured aspects switch
• options.islands.active: false disables all per-island switching

Closes #40

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
10 New issues
0 Accepted issues

Measures
0 Security Hotspots
80.1% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud