feat: add traffic analysis resistance (Phase 7.7 + F8)

- Core: add pad_uniform/unpad_uniform for configurable boundary padding
  and generate_decoy for indistinguishable decoy messages
- Server: add traffic_resistance module with payload padding, timing
  jitter, and background decoy traffic generator (feature-gated)
- P2P: add mesh traffic_resistance module with padded envelopes and
  mesh decoy injection (feature-gated)
- All gated behind --features traffic-resistance
- 22 new tests across core (8), server (4), and P2P (5)

crates/quicproquo-p2p/Cargo.toml

@@ -5,6 +5,9 @@ edition     = "2021"
 description = "P2P transport layer for quicproquo using iroh."
 license     = "MIT"
 
+[features]
+traffic-resistance = []
+
 [lints]
 workspace = true