Fix codegen compatibility with indexmap 2.x in no_std + alloc mode (#20)

This merge request fixes compilation errors when codegen is built with indexmap = { version = "2.11", default-features = false } (no_std + alloc mode).

Problem

• IndexMap in indexmap 2.x requires an explicit hasher parameter when std is disabled.
• IndexMap::new() is not available without the std feature.
• As a result, codegen fails to compile with errors like E0107 and E0599.

Changes

• Added explicit RandomState generic parameter to nested IndexMap usages.
• Replaced IndexMap::new() with IndexMap::with_hasher(RandomState::new()).
• Ensures codegen now builds successfully under both std and no_std + alloc environments.

References

Closes #20 (closed)