test: document compressed batch delta interop upstream limitation

[oferchen]

Summary

• Add test_upstream_compressed_batch_self_roundtrip interop test that verifies upstream rsync can read its own compressed delta batch files (using --compress-choice=zlib workaround), and that oc-rsync can also read them
• Document the upstream rsync 3.4.1 limitation where the batch format does not record which compression algorithm was used - read-batch always assumes CPRES_ZLIB (compat.c:194-195), but write-batch may auto-negotiate zstd on builds with SUPPORT_ZSTD
• Add documentation across batch_support.rs, replay.rs, compressed_writer.rs, flags.rs, and shell test scripts explaining the design decision and upstream limitation
• Add unit test verifying do_compression is always false in oc-rsync batch context regardless of compression config

Background

Task #1705 investigated "WARNING: failed verification" errors when reading compressed batch files with delta transfers. The investigation found this is an upstream rsync 3.4.1 limitation, not an oc-rsync bug:

1. Upstream tees raw compressed wire bytes to the batch file and sets do_compression=true in stream flags
2. The batch format does not record which compression algorithm was used
3. On read-batch, parse_compress_choice() maps do_compression=1 to CPRES_ZLIB regardless of the actual algorithm
4. If the original transfer auto-negotiated zstd, the batch file contains zstd data but the reader uses a zlib decoder

oc-rsync avoids this entirely by recording uncompressed data in batch files (do_compression=false), making batch files portable across all compression backends.

Test plan

• CI passes: fmt+clippy, nextest, Windows, macOS, Linux musl
• New interop test upstream-compressed-batch-self-roundtrip passes
• Existing compressed-batch-delta-interop test continues to pass
• New unit test batch_context_do_compression_false_regardless_of_compress_config passes