fix: var_temp indices are no longer referenced in call assignment

[Angus-Bethke-Bachmann]

Changed

• VAR_TEMP indices are no longer referenced in call assignment

Testing

• Added codegen tests to verify positional assignment for named and implicit parameters when VAR_TEMP interrupts position in struct
• Added lit test to ensure values are assigned correctly and that no LLVM error is present

[mhasel]

I found two remaining edge cases - I've pushed failing tests that showcase them.

1. function_block_with_var_temp_should_compile_when_output_has_direct_access - panics at expression_generator.rs:942, where build_struct_gep still uses the raw position.
2. function_block_with_var_temp_before_input_should_compile - compiles but emits i32 1 instead of i32 0 for the GEP of an input parameter declared after a VAR_TEMP block.

The second test includes an inline snapshot of the correct expected IR so the failure shows a clear diff.

To verify, please also add LIT tests for both cases.