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/prerm.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# llmsproxy core package prerm
+set -e
+
+if command -v systemctl >/dev/null 2>&1; then
+    # stop the service if this package's unit is the one installed
+    if systemctl list-unit-files llmsproxy.service >/dev/null 2>&1 && \
+       systemctl is-active llmsproxy.service >/dev/null 2>&1; then
+        systemctl stop llmsproxy.service >/dev/null 2>&1 || true
+        systemctl disable llmsproxy.service >/dev/null 2>&1 || true
+    fi
+    systemctl daemon-reload >/dev/null 2>&1 || true
+fi
+exit 0