feat(contract): add multi-asset reward support

Contract/earn-quest/src/assets.rs

@@ -0,0 +1,44 @@
+use soroban_sdk::{contracttype, Address, Env, Symbol};
+
+use crate::errors::Error;
+
+/// Supported asset types for multi-asset quest rewards
+#[contracttype]
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum AssetType {
+    /// Native Stellar XLM token
+    Native,
+    /// USDC stablecoin
+    Usdc,
+    /// Custom project or community token
+    Custom,
+}
+
+/// Configuration for a supported reward asset
+#[contracttype]
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct AssetConfig {
+    /// On-chain address of the token contract
+    pub address: Address,
+    /// Classification of the asset
+    pub asset_type: AssetType,
+    /// Minimum allowed reward amount in the token's base unit
+    pub min_amount: i128,
+}
+
+/// Return the canonical symbol string for a given asset type
+pub fn asset_symbol(env: &Env, asset_type: &AssetType) -> Symbol {
+    match asset_type {
+        AssetType::Native => Symbol::new(env, "XLM"),
+        AssetType::Usdc => Symbol::new(env, "USDC"),
+        AssetType::Custom => Symbol::new(env, "CUSTOM"),
+    }
+}
+
+/// Validate that a proposed reward amount satisfies the asset's minimum requirement
+pub fn validate_reward_amount(config: &AssetConfig, amount: i128) -> Result<(), Error> {
+    if amount <= 0 || amount < config.min_amount {
+        return Err(Error::InvalidRewardAmount);
+    }
+    Ok(())
+}

Contract/earn-quest/src/lib.rs

@@ -3,6 +3,7 @@
 use soroban_sdk::{contract, contractimpl, Address, BytesN, Env, Symbol};
 
 mod admin;
+pub mod assets;
 mod errors;
 mod escrow;
 mod init;