feat: add difficulty setting, shared inventory fixes, completed run config

[Niravanaa]

Proposed Changes:

Adds difficulty selection, persistent run save/restore, shared inventory reliability fixes, completed run mechanics display, configurable database naming, and game mode enforcement for lobby and training worlds. Also addresses pre-commit Javadoc violations across several service files.

Details:

• Added Easy / Normal / Hard difficulty selector to the prep GUI; difficulty is applied to the active Minecraft world and recorded with each completed run
• Added persistent save/restore flow: participants vote unanimously with /challenge saverun, snapshotting full player state (position, inventory, health, XP, potion effects) to disk; admins restore with /challenge restore; run timer correctly excludes lobby time between save and restore
• Fixed shared inventory sync timing: inventory click, drag, and several item-interaction events (buckets, boats, block place) now defer the sync by one tick so Bukkit completes the mutation before state is broadcast to other participants
• Fixed un-equipping armor not appearing in other players' inventories due to incorrect per-target armor subtraction; fixed two-set armor disappearance caused by sync firing before wearable detect
• Fixed cursor-held item vanishing when another player edits the shared inventory
• Fixed right-click drag not immediately updating other players' inventories
• Added enabled mechanics list to each completed run entry in the run history screen
• Added configurable database filename (records.db-name in config) for safe dev/test separation from production run history
• Lobby and training worlds now enforce Adventure game mode on entry; bridge training challenge overrides to Survival; game mode is restored on lobby re-entry
• Natural regen block (HEALTH_REFILL off) now only cancels saturation-based regen; Regeneration status effect from potions and beacons remains active
• Half-heart max health is now restored when a player enters a lobby world via world change, not only on join
• Resolved all Javadoc / Checkstyle violations (15 errors across 5 files, plus missing package-info.java)

Related Issues:

• Resolves Difficulty Setting #3
• Resolves Shared Inventory & Armor Sync Bugs #4
• Resolves Completed Run Mechanics Display #5
• Resolves Configurable Test Database #6
• Resolves Persistent Speedrun Save and Restore #7

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.50331% with 268 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ev/deepcore/challenge/ChallengeSessionManager.java	3.92%	98 Missing ⚠️
...hallenge/inventory/SharedInventorySyncService.java	42.66%	39 Missing and 4 partials ⚠️
...epcore/challenge/session/SavedRunStateService.java	85.50%	18 Missing and 11 partials ⚠️
...nventory/InventoryMechanicsCoordinatorService.java	58.06%	18 Missing and 8 partials ⚠️
...ava/dev/deepcore/challenge/ui/PrepGuiRenderer.java	68.00%	10 Missing and 6 partials ⚠️
...deepcore/challenge/session/PrepGuiFlowService.java	30.76%	5 Missing and 4 partials ⚠️
...allenge/session/SessionPlayerLifecycleService.java	76.47%	3 Missing and 5 partials ⚠️
...v/deepcore/challenge/training/TrainingManager.java	33.33%	7 Missing and 1 partial ⚠️
...e/challenge/session/PrepGuiCoordinatorService.java	45.45%	5 Missing and 1 partial ⚠️
...va/dev/deepcore/challenge/ChallengeDifficulty.java	75.00%	3 Missing and 1 partial ⚠️
... and 11 more

Files with missing lines	Coverage Δ	Complexity Δ
.../java/dev/deepcore/challenge/ChallengeRuntime.java	100.00% <ø> (ø)	6.00 <0.00> (ø)
...eepcore/challenge/ChallengeRuntimeInitializer.java	100.00% <100.00%> (ø)	6.00 <0.00> (ø)
...v/deepcore/challenge/command/ChallengeCommand.java	84.61% <ø> (ø)	5.00 <0.00> (?)
...challenge/command/ChallengeCoreCommandHandler.java	85.65% <100.00%> (ø)	69.00 <0.00> (?)
...challenge/command/ChallengeLogsCommandHandler.java	84.54% <ø> (ø)	33.00 <0.00> (?)
...a/dev/deepcore/challenge/command/LobbyCommand.java	100.00% <ø> (ø)	4.00 <0.00> (?)
...epcore/challenge/session/RunCompletionService.java	80.32% <100.00%> (+2.14%)	13.00 <3.00> (ø)
...challenge/session/RunHealthCoordinatorService.java	88.28% <100.00%> (+1.01%)	35.00 <0.00> (+2.00)
...deepcore/challenge/session/RunProgressService.java	90.59% <100.00%> (+10.79%)	43.00 <5.00> (+7.00)
...pcore/challenge/session/SessionFailureService.java	73.91% <100.00%> (+1.18%)	9.00 <0.00> (+1.00)
... and 26 more

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