[omrzk]

The heapless feature promises allocation-free formatting during serialization, but the serializers only select their existing stack-buffer path when the legacy ser_as_str feature is enabled. With heapless alone they call Serializer::collect_str, whose default implementation allocates a String.

Select the existing buffer path for either feature. This covers Ipv4Net, Ipv6Net and their IpNet wrapper while preserving binary serialization and the legacy feature. Fixes #73.

The regression measures allocations while a nonallocating serde_test serializer checks the exact emitted strings, including maximum-width addresses. On the original source, heapless alone records allocations and fails; the legacy serde,ser_as_str control passes with zero allocations.

With the fix, cargo test --features heapless, cargo test --features serde,ser_as_str, cargo test --all-features, and cargo test --no-default-features --features heapless each pass 37 unit tests, the allocation regression and 56 doctests. cargo clippy --all-features --all-targets, focused test formatting and git diff --check pass. Clippy reports existing source warnings, also present on a clean-master control. These checks establish allocation-free formatting with the test serializer; other serializers may allocate themselves.