fix: separate common and campaign error discriminants

[Danservfinn]

Summary

• moves shared common::ErrorCode discriminants into a reserved 1000..=1099 workspace range
• documents the campaign-local 1..=999 and common shared ranges in code comments and deployment notes
• adds a regression test that fails if common and campaign contract-error codes collide again

Closes #41

Validation

• cargo test -p orbitchain-campaign error_discriminant_tests -- --nocapture
• cargo test -p orbitchain-common
• git diff --check
• grep summary: common/src/lib.rs codes 1000..1003; campaign/src/types.rs codes 1..80

[Alqku]

Hi @Danservfinn 👋 — thanks for putting this up. Issue #41 isn't currently assigned to anyone in this repo, so I can't merge under our contributor-matches-assignee policy on this pass. Could you ask a maintainer to assign #41 to you? Once that's done I'll re-confirm CI is still green and merge ✅