fix: XDP unsafe loader + Redis sync; enforce 250-line/4-file layout limits

- xdp: CString for if_nametoindex (was UB), real detach via prog fd
  (fabricated borrow_raw(-1) silently never detached), SAFETY comments,
  saturating expiry math; +5 unit tests
- redis: real pubsub reconnect with exponential backoff (was sleep+return);
  KEYS -> SCAN in heartbeat sweep
- ci: cargo test --all-features, repo-gates job — module size gate
  (scripts/check_module_size.sh, ratchet baseline) + default-secrets grep
- refactor src/ to <=250 LOC/file, <=4 .rs/dir without behavior change;
  thin bins (rampart.rs 330 -> 6 LOC), new app/, subnet/, intel/,
  profile/, prefix/, challenge/, filter/, probe/, inventory/, metrics/, node/
- docs: TODO v5.0 (status refresh, new rules, findings backlog),
  README quickstart now matches real binaries
- verify: fmt/clippy -D warnings/test --all-features (164 tests)/clang XDP green

.module_size_baseline

@@ -0,0 +1,3 @@
+# Module size ratchet baseline — read by scripts/check_module_size.sh
+# Oversized files: plain path. Overcrowded directories: path with trailing /.
+# Regenerate after a refactor: bash scripts/check_module_size.sh --update-baseline