feat: traffic intel hot path, SYN RST-challenge, XDP environment diagnostics

Traffic Intel (was dead code, now wired):
- TrafficHook in listener accept path: cps/pps windows -> AttackDetector
- auto-ban IPs below reputation threshold under attack ([detect.autoban])
- AlertDispatcher: webhook on attack state transition only (dedup), metrics
  AUTO_BANS_TOTAL / INTEL_* ; [detect.alert].webhook_url

XDP SYN RST-challenge (Oubliette pattern, off by default):
- G_SYN_CHALLENGE_ENABLED=0: kernel replies bad-ACK SYN-ACK via XDP_TX,
  spoofed sources stay silent, live clients answer RST with secret echo ->
  challenge_verified (LRU, sliding TTL); brute-force of marker impossible
- maps challenge_verified/challenge_pending, STAT_CHALLENGE_*, all logic
  in xdp/core/syn_challenge.h (221 lines)

XDP diagnostics (src/xdp/diagnostics.rs):
- EnvironmentReport: kernel version/BTF/driver->AttachMode verdict,
  fail-fast before load on unsupported kernels; wired into  CLI
- SystemProbe trait for kernel-less testing

fix: .gitignore 'bin/' matched src/bin/ — rampart.rs was never committed

cargo build/clippy(-D warnings, --features xdp)/test green: 107 tests

xdp/core/stats.h

@@ -20,6 +20,9 @@ static __always_inline void inc_whitelist(void)    { inc_stat(STAT_WHITELIST); }
 static __always_inline void inc_blacklist(void)    { inc_stat(STAT_BLACKLIST); }
 static __always_inline void inc_syn_throttle(void) { inc_stat(STAT_SYN_THROTTLE); }
 static __always_inline void inc_rate_limit(void)   { inc_stat(STAT_RATE_LIMIT); }
+static __always_inline void inc_chal_sent(void)     { inc_stat(STAT_CHALLENGE_SENT); }
+static __always_inline void inc_chal_verified(void) { inc_stat(STAT_CHALLENGE_VERIFIED); }
+static __always_inline void inc_chal_failed(void)   { inc_stat(STAT_CHALLENGE_FAILED); }
 static __always_inline void inc_pass(void)         { inc_stat(STAT_PASS); }
 static __always_inline void inc_drop(void)         { inc_stat(STAT_DROP); }