fix: define PRECISION in gathera_common, import across workspace

[Alqku]

Summary

storage.rs referenced crate::cross_chain::PRECISION which is pub(crate) in cross_chain.rs. This caused a compile error and accidental coupling between modules. The fix moves PRECISION to gathera_common as the single source of truth.

Changes

• contract/common/src/lib.rs: add pub const PRECISION: i128 = 1_000_000_000 with a doc-comment explaining its role.
• contract/contracts/src/storage.rs: import gathera_common::PRECISION and replace both crate::cross_chain::PRECISION references in update_reward.
• contract/contracts/src/cross_chain.rs: remove the local pub(crate) const PRECISION and re-import from gathera_common for any remaining internal use.

Testing

• cargo check --workspace exits 0 with no undefined-constant errors.
• update_reward arithmetic is unchanged; only the import source differs.

Closes #513