feat(rust): add schema-aware incremental item writer (#127)

[nth-bailey]

Summary

Closes #127.

Adds a schema-aware incremental item producer (write_<root>_items) for plain sequence root elements with one repeated child collection. Instead of requiring a complete materialized object tree in memory, the producer consumes items lazily from an iterator and streams directly to any std::io::Write sink with bounded O(1) memory.

Key Changes

• Rust Codegen (polyxml-core): Emits pub fn write_<root>_items for eligible sequence roots, handling root opening/closing tags, root namespaces, child item encoding, text escaping, and minOccurs/maxOccurs occurrence bounds.
• Verification Harness (scripts/verify_incremental_writer.py): Tests both borrowed and owned generated models, verifying minimum/maximum occurrences, empty optional collections, sink failures, lazy iterator consumption, and independent libxml2 streaming XSD validation.
• Documentation & Benchmarks (docs/benchmarks/incremental-writer.md): Records peak resident memory across 1K, 100K, and 1M items (~2.5 MiB flat RSS from 74 KB to 76.8 MB XML).
• Language Guide (docs/languages/rust.md): Documents the incremental item writer API, distinguishing it from whole-model sink streaming.