The implicit address-of that gives `*T` params reference semantics only
fired for plain identifiers (`mut(v)`). For a field-access / index /
deref lvalue (`make_move(self.board, m)`, `mut(w.s)`), the branch was
skipped: the arg was loaded into a temporary and the callee mutated a
throwaway copy — silent data loss, with the type check satisfied through
the temp so no diagnostic fired.
Now compound lvalues auto-ref too: take the real lvalue address via
`lowerExprAsPtr`, normalizing the "place" ref to `*T` exactly as
`@field_access` does. Mutations through the pointer are now visible to
the caller, matching the identifier case.
Regression: examples/255-autoref-compound-lvalue.sx.
49dc622234