`(&mut t[i][j]).as_mut_ptr()` crashes the compiler

fn main() -> i32 {
  let mut c = Tensor<f32>([2, 2]);
  c[0][0] = 1.0;
  let pc = (&mut c[0][0]).as_mut_ptr();
  return 0;
}

[flat-codegen] declined main: no flat lowering for the MethodCall expression [unsupported-expr(MethodCall)]
[flat-codegen] program outside the flat subset; using the AST path
Vx Compiler Internal Error: panicked at src/codegen/lower/expr.rs:2727:13:
Function as_mut_ptr not found

The checker accepts as_mut_ptr on a borrow of a tensor element and gives it a type; the AST lowering then looks for a function of that name, does not find one, and panics. Either the checker should reject the receiver or the lowering should handle it, but a program a user can write should not reach an unreachable-style panic — same shape as the mlir! parse panic in Vx#414.

as_mut_ptr on the tensor itself is handled and yields *mut Tensor<f32, [2, 2]>; only the element-borrow receiver falls through.

Found while probing the aliasing premise in the issue above.

Start by running the provided Vx reproduction, then inspect src/codegen/lower/expr.rs around the reported panic and the checker handling for as_mut_ptr on a borrowed tensor element. Done means this valid program no longer reaches the compiler panic: it is either rejected by the checker or lowered successfully, with coverage for the receiver distinction.