feat(packaging): core distribution packages (deb + rpm + tar.gz) via nfpm

Previously only the Electron GUI had installers; server admins had to build the
core by hand (`make build` -> bare binary). This adds a first-class server
distribution:

- `make core-dist` -> packaging/core-dist.sh -> cmd/build/dist/
    llmsproxy_<ver>_amd64.deb        (systemd unit + adapters + example config)
    llmsproxy-<ver>.x86_64.rpm
    llmsproxy-<ver>-linux-amd64.tar.gz
- nfpm config (packaging/nfpm.yaml): binary to /usr/bin, systemd unit with the
  measured memory tuning (GOGC=50, MALLOC_ARENA_MAX=2) baked in, Lua adapters to
  /usr/share/llmsproxy/adapters, repo's config.yaml as the example.
- postinst seeds /etc/llmsproxy/config.yaml on first install and keeps existing
  runtime state across reinstalls; prerm stops the service on removal.

Gotchas handled: nfpm v2 here does not render `{{ .Env.VERSION }}`, so the script
stamps the version into a throwaway config copy; and under `set -o pipefail` the
idiom `strings | grep -q` is broken (grep -q closes the pipe and strings dies on
SIGPIPE -> spurious 141), so the symbol sanity-gate stages strings in a temp file.

README (zh/en) updated: memory figures replaced with production-measured
32-35 MB settled (was 37-42), and the packaging docs now describe core-dist and
the dockerized Windows build.

packaging/llmsproxy.service

@@ -0,0 +1,24 @@
+[Unit]
+Description=LLMSProxy - unified OpenAI-compatible multi-source LLM gateway
+After=network.target
+
+[Service]
+Type=simple
+# Memory tuning (measured, see README "内存占用"):
+#   MALLOC_ARENA_MAX=2 caps glibc per-thread malloc arenas. LuaJIT allocates
+#     through cgo -> glibc malloc, and glibc defaults to 8*nproc arenas, so every
+#     OS thread that touches malloc reserved its own ~1 MB arena that is never
+#     returned. Measured: 8-12 arenas -> 0.
+#   GOGC=50 halves the Go heap growth target. On its own it does NOT help (the
+#     saved heap is immediately eaten by more glibc arenas); combined with
+#     MALLOC_ARENA_MAX it cut settled RSS by ~19%. This gateway is I/O bound, so
+#     the extra GC cycles are free.
+Environment=GOGC=50
+Environment=MALLOC_ARENA_MAX=2
+ExecStart=/usr/bin/llmsproxy -config /etc/llmsproxy/config.yaml
+WorkingDirectory=/etc/llmsproxy
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target