test: regression test suite pinning public interface baseline

[MooreTheAnalyst]

Summary

Implements #118 - Regression Test Suite

Adds comprehensive regression tests that pin the exact function signatures, error codes, event topics, and storage key names emitted by the current contract version. Any future refactor that accidentally changes the public interface will be caught before merge.

Changes

• test_regression.rs: Complete regression test coverage including:

  • Error codes: All 32 ChainSettleError variants validated against exact u32 values (1-32)
  • Event topics: All 39 event topics verified for consistent Symbol creation
  • Storage keys: All DataKey variants validated via encode/decode round-trip
  • Enum variants: ShipmentStatus and DataKey variants existence checks
  • Baseline counts: Error codes (32), event topics (39), storage keys (35) tracked to catch schema changes

• .github/workflows/ci.yml: Added regression job:

  • New regression job runs cargo test -- --ignored independently
  • Tests marked #[ignore = "regression"] for explicit triggering

Acceptance Criteria

✓ All event topic Symbols pinned as string definitions
✓ All error code integer values pinned (1-32)
✓ Storage key encoding round-trips verified
✓ Regression suite runs separately via --ignored flag
✓ CI job added to run regression tests independently

Closes #118

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