MinePieceShop

## MinePiece Shop - Economy Rebalance Update

### New Shop Stock
- Added all 39 v14 Devil Fruits to Rayleigh's shop (Legendary tab)
- Added Long Stick (Uncommon, 2,800฿), Perona Umbrella (55,000฿), Dyna Stones (Rare, 100,000฿)
- Added Shield Biscuit (180,000฿), Shield Note (150,000฿), Shield Uta (Epic, 250,000฿)
- Added Long Stick / Perona Umbrella / Shield Uta / Phoenix / Daibutsu / Ope to gambling pools
- Imu's fruit is intentionally not for sale

### Price Rebalance
- Legendary fruits retiered by strength:
  - God tier 1.3M - 2.0M (Yami 2.0M, Gura / Uo Seiryu 1.8M, Ope / Phoenix / Daibutsu 1.6M, Nikyu / Pika 1.5M...)
  - Strong tier 900k - 1.2M
  - Utility tier 400k - 800k
- Farmable / grindable items now resell at 10% instead of 40%: obsidian, crying obsidian, glowstone, sea lantern, ender pearls/eyes, blaze rods/powder, ghast tears, slime/magma, membranes, turtle eggs, arrows, bread/beef, ancient debris, XP bottles, golden apples

### Money Exploit Fixes
- Removed generic 10-100฿ sell fallback - unsellable items now sell for 0฿
- Enchanted books, potions, and NBT-tagged items no longer match the loose sell fallback, must be exact shop matches
- Mob kill belly capped at 500฿, with diminishing returns past 100 HP (sqrt scaling)
- Named mob bonus reduced from 5x to 2x
- Shopkeeper kills give no belly by default
- Belly pouch drops reduced from 100k max to 25k max, no pouches from shopkeepers
- Sell packet: armor/offhand slots blocked, Sell All limited to main inventory (0-35), overflow guards added
- Roulette spam slowed: 1s -> 3s cooldown

### GUI Improvements
- New FRUITS category tab
- Resale % shown in item tooltips
- Purchase confirmation dialog for items >= 250,000฿ (prevents accidental legendary buys)
- Fallback shop list synced with new items

### New Gamerules - Shop NPC Control
- `shopKeepInvulnerable` (default true) - toggle damage to Rayleigh / Perona / Imu
- `shopAllowDespawn` (default true) - toggle timeout / range despawning
- `shopKillRewards` (default false) - toggle belly / drops from killing shopkeepers
- `shopSpawnCooldownSec` (default 1200) - control respawn cooldown for all 3 NPCs (Imu uses 3x)