fix(cgen): fix address-of and as_cast pointer codegen #6

### Summary This PR fixes a C code generation issue where combining the address-of (`&`) operator and `as_cast` expressions (specifically during smartcasts) results in incorrect pointer casting and dereferencing syntax in the generated C output. ### Details of Changes In `vlib/v/gen/c/cgen.v`: 1. **`Gen.expr`**: - Enhanced the address-of (`&`) operator codegen path. When the right-hand side is an `AsCast` expression, we check if it evaluates to a pointer casting using the new `as_cast_will_use_ptr()` helper. If so, we avoid writing incorrect prefix operators (like double address-of or invalid dereference syntax). 2. **`Gen.write_as_cast_call_start`**: - Added a check for `g.is_amp` (address-of context) to ensure the compiler generates `(Type*)` pointer casts rather than standard value casts when smartcasting is performed in an address-of context. 3. **`Gen.as_cast_will_use_ptr`**: - Introduced this helper method to accurately determine if an `AsCast` node will result in a pointer-based representation under the hood (e.g. casting interfaces or sum types with differing base types). ### Checklist - [x] I have formatted my V files with `v fmt -w`. - [x] All existing tests pass. ### Tests Run Rebuilt the V compiler and verified that all compiler and cgen tests pass: ```bash ./v -g -keepc -o ./vnew cmd/v ./vnew -silent test vlib/v/gen/c/ ``` All tests passed successfully on local machine.