feat(paper): add material item economy provider

[TheSilentPro]

Summary

Adds an optional material-backed item economy provider without changing the existing Vault purchase path.

Configuration

enabled: true
provider: item

item:
  match: material
  material: DIAMOND
  singular: Diamond
  plural: Diamonds

prices:
  any-head: 2

Behavior

• adds provider: item alongside the existing vault provider;
• requires explicit item.match: material for the initial implementation;
• supports one globally configured Bukkit material as currency;
• matches by material only, regardless of item metadata;
• consumes only normal player storage inventory contents (not armor/offhand/cursor);
• performs a full availability pass before mutating inventory, so insufficient purchases remove nothing;
• supports currency spread across multiple stacks;
• formats prices as 1 Diamond / 2 Diamonds (configurable singular/plural labels);
• keeps the existing per-head → custom-category → player/category → fallback price precedence;
• requires whole-number prices for item economy and disables item economy on startup if fractional configured prices exist;
• rejects fractional prices entered through the existing admin price editors while item economy is active;
• leaves Vault provider behavior unchanged;
• automatically works with the existing GUI price lore introduced in feat(paper): display economy prices in head lore #103.

Includes focused unit tests for formatting, whole-number validation, multi-stack withdrawal, and atomic insufficient-funds behavior.