test(protocol): adversarial stream corpus for parser robustness (#1195)

[oferchen]

Summary

• Add crates/protocol/tests/adversarial_stream_corpus.rs, a curated catalogue of malformed, truncated, and edge-case byte streams that exercise the multiplex reader, legacy @RSYNCD: handshake parser, varint and fixed-int decoders, delta token decoder, file-list name decoder, and filter-rule wire decoder.
• Each corpus entry pins an expected outcome (Ok, specific io::ErrorKind, or NoPanic) and carries an inline comment describing the regression it would catch.
• Three reusable runners assert outcomes against the recorded expectations and run every case through catch_unwind to guarantee parsers never panic, crash, OOM, or hang.
• Designed as the static regression seed for live interop fuzzing (Prevent module burst from raising daemon bwlimit #1196). New entries are added by appending to the relevant static array.

Coverage

• Truncated multiplex headers (1- and 3-byte) and oversized 16 MiB length claims with no payload.
• Tags below MPLEX_BASE and unmapped message codes.
• Multiplex header bytes embedded inside another header's payload (asserts no recursive descent).
• Invalid UTF-8 inside MSG_INFO payloads (multiplex layer must remain transparent).
• Zero-length MSG_DATA frames (valid flush boundary).
• Truncated legacy @RSYNCD: greetings, malformed subprotocol markers, and saturating numeric overflow.
• Negotiation prologue detector replayed for every truncation of a full legacy banner.
• Delta token decoder: empty stream, truncated read_int, end marker, negative block match, i32::MIN block match.
• Internal delta opcode decoder: negative literal length rejection.
• Varint decoder: overflow leading byte, empty stream, truncated 4-byte extension.
• Legacy 4-byte read_int and longint extension truncation.
• File-list name decoder: embedded NUL bytes preserved, truncated long-name suffix surfaces EOF.
• Filter list wire decoder: negative length, oversized length, invalid UTF-8 body, terminator-only stream, embedded NUL in pattern.

Test plan

• cargo nextest run -p protocol --all-features -E 'test(adversarial_stream_corpus)' passes on Linux, macOS, and Windows.
• Full cargo nextest run --workspace --all-features remains green.