refactor(workspace): extract model types

[hongjr03]

Summary

• add a workspace-model crate for shared source file, source root, and project config types
• keep HIR base_db paths as compatibility re-exports
• redirect project-model to depend on workspace-model instead of hir

Validation

• cargo fmt
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace