test(fuzz): rsyncd.conf, auth response, incremental flist fuzz targets (FCV-3 followups)

[oferchen]

Summary

• Three new fuzz targets closing high-priority gaps from PR docs(audit): FCV-3 protocol-parsing fuzz coverage gaps (#2316) #4407 (FCV-3 audit):
  • rsyncd_conf (pre-auth admin attack surface) drives RsyncdConfig::parse.
  • auth_response (pre-auth) drives both verify_client_response (challenge-reply verifier) and SecretsFile::parse (admin secrets file). A one-byte selector splits the corpus between the two surfaces.
  • incremental_flist (post-auth, pre-transfer) drives StreamingFileList::next_ready plus IncrementalFileList::finalize under both legacy (protocol 28) and INC_RECURSE (protocol 30 + CF_INC_RECURSE) wire modes.
• Each target ships with a minimal seed corpus under fuzz/corpus/<target>/seed_basic.
• Wires daemon into fuzz/Cargo.toml as a new path dep (default features off).
• Trailing rustfmt cleanups in two existing fuzz harness files surfaced by cargo fmt over the crate.

API substitutions made

The audit prompt sketched the entry points as daemon::config::parse_conf, daemon::auth::verify_response, and protocol::flist::incremental::parse_segment. The actual public APIs differ slightly; nearest analogs used:

Suggested	Used
daemon::config::parse_conf(&str)	daemon::rsyncd_config::RsyncdConfig::parse(input: &str, path: &Path)
daemon::auth::verify_response(&[u8])	daemon::auth::verify_client_response(secret: &[u8], challenge: &str, response: &str, protocol_version: Option<u8>) plus daemon::auth::SecretsFile::parse(&str)
protocol::flist::incremental::parse_segment(&[u8])	protocol::flist::StreamingFileList::new(FileListReader, Cursor<&[u8]>).next_ready() plus IncrementalFileList::finalize()

The goal is bytes-in fuzz coverage of the named attack surface, which is preserved.

Test plan

• cargo fmt clean on the three new files (verified via targeted rustfmt).
• All three targets register as new [[bin]] entries in fuzz/Cargo.toml.
• All three targets compile via cargo +nightly fuzz build <target> (CI-only).
• Seed corpora present under fuzz/corpus/{rsyncd_conf,auth_response,incremental_flist}/seed_basic.