Refactor specialize.rs for efficiency and clarity

[seth-schultz]

Summary

Refactors the generic function specialization pass for improved performance and code clarity.

Changes

• Fix O(n²) RIR traversal: Store rir_params_start/len in FunctionInfo for O(1) param lookup
• Remove redundant HashMap: Eliminate intermediate name_map - pass specializations directly
• Use entry API: Avoid unnecessary .clone() on type_args in collect_specializations
• Simplify iteration: Use param_arena.iter() instead of manual 4-way zip
• Add debug assertion: Catch type substitution failures in debug builds
• Move imports to top: Follow codebase convention for Entry import

Result

• 280 lines (down from 334, -16%)
• Same functionality, cleaner code
• All tests pass

Testing

• All 1341 spec tests pass
• All 445 unit tests pass
• Generic functions and anonymous struct types work correctly