refactor: clap CLI parser, safety docs, dead code docs, cross-refs

Audit items 6, 8, 9, 10:

#6 - Relay CLI: replaced 154-line manual parse_args() with clap derive
     (13 flags/options preserved, auto --help, --version from build hash)
#8 - wzp-native: added # Safety docs to all 3 unsafe extern "C" fns
#9 - wzp-crypto: documented x25519_static_secret/public as reserved for
     future static-key federation auth (not dead code, intentionally unused)
#10 - Cross-references between quality.rs ↔ dred_tuner.rs module docs

368 tests passing, 0 regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/Cargo.toml

@@ -20,6 +20,7 @@ bytes = { workspace = true }
 serde = { workspace = true }
 toml = "0.8"
 anyhow = "1"
+clap = { version = "4", features = ["derive"] }
 reqwest = { version = "0.12", features = ["json"] }
 serde_json = "1"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }