feat(asset): add AssetAmount wrapper type

[giwaov]

Summary

Introduces a validated AssetAmount(u64) newtype that encapsulates the max-amount invariant (2^63 - 2^31) at construction time, replacing the raw u64 field inside FungibleAsset.

Changes

New: AssetAmount (asset_amount.rs)

• MAX constant equal to 2^63 - 2^31
• new(amount: u64) -> Result<Self, AssetError> with validation
• as_u64() accessor
• From<u8>, From<u16>, From<u32> (infallible small-type conversions)
• TryFrom<u64> (delegates to new)
• From<AssetAmount> for u64
• Display, Serializable, Deserializable
• Unit tests for all of the above

Modified: FungibleAsset (fungible.rs)

• Internal amount field changed from u64 to AssetAmount
• FungibleAsset::MAX_AMOUNT kept as a backward-compatible u64 alias pointing to AssetAmount::MAX
• new() delegates validation to AssetAmount::new()
• amount() still returns u64 via as_u64() to minimize downstream churn
• add() / sub() updated to work through AssetAmount
• Serialization writes self.amount.as_u64()

Modified: mod.rs

• Added mod asset_amount; pub use asset_amount::AssetAmount;

Testing

• Full workspace cargo check passes
• Existing FungibleAsset tests are compatible (no API surface change for callers)

Closes #2532