[omrzk]

Network iterators currently cannot be reversed without collecting the addresses first. Implement DoubleEndedIterator for the IPv4, IPv6 and enum iterators, so callers can use .rev() and mix front/back reads without allocating. Fixes #233.

Both ends share the remaining range and stop when they meet. Tests cover all 16 front/back consumption patterns for four-address networks, reversed order, singleton zero/maximum addresses, highest IPv4 addresses, and both ends of the full IPv6 range. The new tests fail to compile on the original code because the traits and next_back methods are absent.

Validation: cargo test --all-features passes 80 unit tests, 5 existing integration tests, 5 new tests and 28 doctests (one existing expensive unit test stays ignored). After adding the documentation example, all 29 doctests pass. cargo build, cargo doc, cargo clippy --all-features, targeted rustfmt checks and git diff --check pass. Repository-wide cargo fmt --check reports existing formatting in the benchmarks, schema declaration and JSON tests; none of those lines are changed here.

Existing network bounds and size-hint behavior remain as implemented upstream; this does not address the separate /0 work in #179.