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-proto/src/dred_tuner.rs

@@ -10,6 +10,10 @@
 //! prediction): when jitter variance spikes >30% over a 200 ms window — typical
 //! of Starlink satellite handovers — it temporarily boosts DRED to the maximum
 //! allowed for the current codec before packets actually start dropping.
+//!
+//! See also: [`crate::quality`] for discrete tier classification that drives
+//! codec switching. DredTuner operates within a tier, adjusting DRED
+//! parameters continuously based on live network metrics.
 
 use crate::CodecId;
 

crates/wzp-proto/src/quality.rs

@@ -1,3 +1,5 @@
+//! See also: [`crate::dred_tuner`] for continuous DRED tuning within a tier.
+
 use std::collections::VecDeque;
 use std::time::{Duration, Instant};