REALM-004: Replace realm nesting if-chain with 6x6 lookup table

[Raulgooo]

Summary

is_realm_nesting_legal in src/typecheck.c uses an if-chain. Replace with a static const bool 2D array indexed by [outer][inner] per spec §8.

Scope

1. Read spec §8 nesting matrix and extract all 36 legal/illegal combinations
2. Implement as static const bool realm_nesting_legal[6][6] (indexed by MemoryRealm enum values)
3. Replace the if-chain with a single table lookup
4. Any promotion rules with conditional logic (e.g., unsafe block overrides) must be flagged in a comment as requiring code outside the table

Acceptance Criteria

• All existing realm tests still pass
• Table matches spec §8 matrix exactly
• If-chain removed

Related

• Spec §8