fix(lower): diagnose .* on a non-pointer instead of codegen panic

`lowerDerefExpr` left the deref's result type `.unresolved` when the
operand wasn't a pointer (e.g. a stale `value.*` after a parameter
changed from `*T` to `T`), and emitted the `.deref` anyway. That
unresolved type slipped through to emit_llvm's "unresolved type reached
LLVM emission" panic with no source location.

Now it emits a clean diagnostic at the deref site
("cannot dereference with `.*`: 'T' is not a pointer") and recovers.
Regression: examples/254-deref-non-pointer-reject.sx.

tests/expected/254-deref-non-pointer-reject.exit

@@ -0,0 +1 @@
+1

tests/expected/254-deref-non-pointer-reject.txt

@@ -0,0 +1,5 @@
+error: cannot dereference with `.*`: 'Point' is not a pointer
+  --> /Users/agra/projects/sx/examples/254-deref-non-pointer-reject.sx:12:10
+   |
+12 |     q := p.*;          // ERROR: `p` is a Point value, not a pointer
+   |          ^