[W5][SC][M] Add contract panic-code mapping docs and invariant assertions

[kike-alt]

ID: W5-3C-SC-001
Track: SC
Phase: 3C
Difficulty: intermediate
Priority: P1

Description

Document panic/error code mapping and assert invariant-linked codes remain stable.

Acceptance Criteria

• Define implementation plan aligned to scope boundaries
• Implement and verify changes in soroban/contracts/core_game/src/lib.rs
• Implement and verify changes in soroban/contracts/rewards/src/lib.rs
• Add/adjust tests or validation artifacts for the changed surface
• Update docs/notes if behavior or operations change

Relevant Files/Modules

• soroban/contracts/core_game/src/lib.rs
• soroban/contracts/rewards/src/lib.rs
• soroban/contracts/achievements/src/lib.rs
• docs

Dependencies / Blocked By

• [W5][SC][M] Define and lock Soroban event schema fixtures across foundation contracts #577
• [W5][SC][L] Build cross-contract integration harness for foundation modules #582

Validation Requirements

• contract tests fail on panic-code drift from documented mapping