init(server/game): rewrite the server in c++

third_party/ikcp/README.md

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+# ikcp (patched)
+
+Upstream: https://github.com/skywind3000/kcp (public domain / MIT).
+
+Star Rail uses a 28-byte KCP header instead of the stock 24: a 4-byte session
+`token` follows `conv`, both little-endian. The vendored copy is patched for it:
+
+- `IKCP_OVERHEAD` 24 -> 28
+- `token` field on `IKCPSEG` and `IKCPCB`
+- `ikcp_encode_seg` / `ikcp_input` write and verify it
+- `ikcp_settoken(kcp, token)` to set it after `ikcp_create`
+
+Handshake packets carry conv/token as **big-endian** u32; the KCP header carries
+them little-endian. That byte reversal is what the real client expects.