Refactor `unwrap()` and type signatures in `src/codegen/generator.rs` to use `Result`

Description

During the codegen panic cleanup pass, over 340 unwrap() calls were safely removed from src/codegen/lower/. However, some heavy .unwrap() usages remain in src/codegen/generator.rs, primarily inside coerce_type and lower_type.

Changing these functions to return Result<..., LowerError> currently cascades into over 100+ type mismatch errors across the compiler because they are called deep within type inference and static type instantiation where Result propagation cannot easily be bubbled up without a larger architectural shift in the compiler driver.

Action Items

• Refactor coerce_type and lower_type in generator.rs to safely return Result.
• Propagate these errors correctly through callers that currently expect a raw Type.
• Eliminate the remaining compiler panics during MLIR AST lowering in generator.rs.

Start in src/codegen/generator.rs by reading coerce_type and lower_type, then trace their callers through type inference and static type instantiation. The issue notes that Result propagation currently causes more than 100 type mismatches, so map that call chain before changing signatures. Done means these functions propagate LowerError and compiler panics during MLIR AST lowering are eliminated.