feat!: universal redesign — drop Minecraft stack, single-crate architecture

- remove Java plugins (velocity/paper), dashboard, all MC-specific code
  (handshake, death_code, varint, hostname-HMAC); available in history pre-v0.2
- merge crates/* into one package with src/bin/{rampart,rampart-manager,rampart-cli}
- ProtocolHandler trait + registry (no implementations yet), universal PoW kept
- XDP: universal L3/L4 filter (xdp/core/) + pluggable hook API (xdp/hooks/),
  fix IPv6 saddr bug; clang build verified
- docs: bilingual knowledge base (docs/kb/: attacks x4, defense-levels,
  practice x3), rewrite README/architecture for universal concept
- TODO.md v4.0: <=300-line module limit, competitor benchmark section (ref/)
- deploy/CI/docs cleanup: no MC references, new binary names

cargo build/clippy(-D warnings)/test green (55 tests)

tests/pow_roundtrip.rs

@@ -0,0 +1,18 @@
+use rampart::engine::challenge::{Challenge, solve};
+
+#[test]
+fn solver_output_passes_verifier() {
+    let mut challenge = Challenge::generate(3);
+    let nonce = solve(&challenge.challenge_string(), 3).expect("solver must find nonce for difficulty 3");
+    assert!(challenge.verify(&nonce));
+}
+
+#[test]
+fn verifier_rejects_garbage_and_replay() {
+    let mut challenge = Challenge::generate(2);
+    assert!(!challenge.verify("garbage"));
+
+    let nonce = solve(&challenge.challenge_string(), 2).expect("solved");
+    assert!(challenge.verify(&nonce), "first use passes");
+    assert!(!challenge.verify(&nonce), "replay is rejected");
+}