Add initial configuration and placeholder for LevelingCore functionality

Author: AzureDoom

src/main/java/com/azuredoom/levelingcore/LevelingCore.java

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+package com.azuredoom.levelingcore;
+
+// TODO: Implement leveling core functionality to Hytale Server API
+public class LevelingCore {}

src/main/resources/default.yml

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+# LevelingCore configuration
+#
+# =========================
+# Database Configuration
+# =========================
+#
+# Supported JDBC URLs:
+#   H2 (file):      jdbc:h2:file:./data/plugins/levelingcore/levelingcore;MODE=PostgreSQL
+#   MySQL:          jdbc:mysql://host:port/dbname
+#   MariaDB:        jdbc:mariadb://host:port/dbname
+#   PostgreSQL:     jdbc:postgresql://host:port/dbname
+#
+# Notes:
+# - H2 commonly uses empty username/password unless you configured otherwise.
+# - For MySQL/MariaDB/Postgres, set username/password.
+#
+database:
+  jdbcUrl: "jdbc:h2:file:./data/plugins/levelingcore/levelingcore;MODE=PostgreSQL"
+  username: ""
+  password: ""
+  maxPoolSize: 10
+
+# =========================
+# Leveling Formula
+# =========================
+#
+# Supported types:
+#   - EXPONENTIAL: XP floor at level L is baseXp * (L - 1) ^ exponent
+#   - LINEAR:      XP floor at level L is xpPerLevel * (L - 1)
+#   - TABLE:       XP floor at level L is defined in a CSV file
+#   - CUSTOM:      XP floor at level L is defined by a math expression
+#
+# Notes:
+# - XP migration is enabled by default. Set migrateXP to false to disable.
+# - Changing the formula will recompute XP to preserve player levels.
+#
+formula:
+  type: "EXPONENTIAL"
+  migrateXP: true
+  exponential:
+    baseXp: 100.0
+    exponent: 1.7
+    # Maximum level supported by this formula
+    maxLevel: 100000
+  linear:
+    xpPerLevel: 100
+    # Maximum level supported by this formula
+    maxLevel: 100000
+  table:
+    # CSV file relative to the data directory
+    file: "levels.csv"
+  custom:
+    # Expression returns the XP floor for a level.
+    #
+    # Available variables:
+    #   - level      (current level, integer >= 1)
+    #
+    # You may also reference any constants defined below.
+    #
+    # Example:
+    #   exp(a * (level - 1)) * b / c
+    #
+    xpForLevel: "exp(a * (level - 1)) * b / c"
+
+    # Optional constants referenced in the expression
+    constants:
+      a: 0.12
+      b: 100
+      c: 1
+
+    # Maximum level supported by this formula (used for binary search)
+    maxLevel: 100000