Replace @Nullable with @MonotonicNonNull on a lazily-initialized field

Replaces the JSpecify @Nullable on a private, non-final reference field with the Checker Framework @org.checkerframework.checker.nullness.qual.MonotonicNonNull annotation when it is a lazily-initialized-non-null field, so NullAway treats reads of it as non-null and drops the requireNonNull noise a plain @Nullable forces at every read. A field qualifies only if it is private and non-final, declares exactly one reference-typed variable, every assignment has a provably-non-null right-hand side, at least one assignment is outside a constructor, and every null-observation of the field is a lazy-init guard (the condition of an if whose then-branch assigns the field). Because private bounds the analysis to the declaring class and no use handles null gracefully, the upgrade is behavior-preserving. Only Java sources are modified.