test(protocol): golden byte tests for iconv-converted filenames (#1919)

[oferchen]

Summary

• Locks the byte-level wire encoding of filenames that pass through the --iconv pipeline (config build Propagate fallback signal exit codes #1911, sender flist emit Cache proxy authorization headers #1912, receiver flist ingest Ensure fallback binary lookup mirrors Windows PATHEXT #1913) so future regressions in either direction are caught at the byte level.
• Verifies UTF-8 -> ISO-8859-1, KOI8-R, and windows-1252 conversion both ways: sender writes exact remote-charset bytes; receiver decodes them back into UTF-8 disk names. Identity (UTF-8 <-> UTF-8), ASCII passthrough, round-trip, suffix-len-after-iconv, and prefix-compression-after-iconv invariants are all asserted at the byte level.
• Negative cases mirror upstream's IOERR_GENERAL path in flist.c: a Greek alpha source name targeting ISO-8859-1 (sender) and malformed UTF-8 wire bytes (receiver) both surface io::ErrorKind::InvalidData.

Coverage

Direction	Test	Asserts
Sender	golden_sender_utf8_to_latin1_cafe	café -> 63 61 66 e9
Sender	golden_sender_utf8_to_koi8r_cyrillic	файл -> c6 c1 ca cc
Sender	golden_sender_utf8_to_windows1252_angstrom	Ångström.txt -> 12-byte CP1252 form, suffix_len=12
Sender	golden_sender_ascii_passthrough_under_iconv	iconv path leaves ASCII bytes unchanged
Sender	golden_sender_identity_converter_preserves_utf8	identity converter is detected and skips re-encoding
Sender	golden_sender_unmappable_char_fails_conversion	U+03B1 -> ISO-8859-1 fails with InvalidData
Sender	golden_sender_suffix_len_is_post_iconv_for_shrinking_conversion	suffix_len = 4 for café over ISO-8859-1
Sender	golden_sender_suffix_len_is_post_iconv_for_cyrillic	suffix_len = 4 for файл over KOI8-R
Receiver	golden_receiver_latin1_to_utf8_cafe	63 61 66 e9 -> UTF-8 café
Receiver	golden_receiver_koi8r_to_utf8_cyrillic	c6 c1 ca cc -> UTF-8 файл
Receiver	golden_receiver_windows1252_to_utf8_angstrom	12-byte CP1252 -> 14-byte UTF-8
Receiver	golden_receiver_ascii_passthrough_under_iconv	ASCII bytes survive iconv unchanged
Receiver	golden_receiver_invalid_remote_bytes_fail	[0xc3, 0x28] (bad UTF-8) -> InvalidData
Round-trip	golden_round_trip_cafe_via_latin1	café survives ISO-8859-1 hop
Round-trip	golden_round_trip_cyrillic_via_koi8r	файл survives KOI8-R hop
Round-trip	golden_round_trip_angstrom_via_windows1252	Ångström.txt survives windows-1252 hop
Round-trip	golden_round_trip_dir_then_file_compressed_after_iconv	XMIT_SAME_NAME prefix compression operates on post-iconv bytes
Cross-check	golden_sender_wire_equals_raw_remote_bytes_for_cafe	iconv writer output == raw-remote-bytes writer output
Cross-check	golden_receiver_post_iconv_equals_utf8_native_decode	iconv reader output == plain reader output for the equivalent UTF-8 wire

Tests gated on feature = "iconv". Receiver-side and round-trip tests additionally gated on Unix, where OsStr preserves arbitrary byte sequences (required to thread non-UTF-8 wire bytes through the writer for the receiver tests).

Wire-format expectations cross-referenced against upstream rsync 3.4.1 flist.c:

• send_file_entry() lines 1580-1602 - iconvbufs(ic_send, ...) on file->dirname and file->basename before they reach the wire.
• recv_file_entry() lines 738-753 - iconvbufs(ic_recv, ...) after read_sbuf() into thisname, before clean_fname().

Test plan

• cargo nextest run -p protocol --all-features -E 'test(iconv) or test(golden)' -> 443 tests run, 443 passed
• cargo fmt --all -- --check clean
• cargo clippy -p protocol --all-features --tests --no-deps -- -D warnings clean
• CI: fmt+clippy
• CI: nextest (stable) on Linux, Windows, macOS, Linux musl