v0.5.1: static musl builds for OpenWRT (amd64 + arm64)

A user on OpenWRT x86_64 reported the linux release doesn't run there —
root cause was glibc vs musl mismatch (our gnu binary was looking for a
dynamic linker that doesn't exist on router userlands). Add two musl
targets that produce fully static PIE binaries:

- x86_64-unknown-linux-musl  -> mhrv-rs-linux-musl-amd64.tar.gz
- aarch64-unknown-linux-musl -> mhrv-rs-linux-musl-arm64.tar.gz

CI uses the messense/rust-musl-cross docker images (better-maintained
than cargo-zigbuild with a pinned zig, which has version regressions
on the ar wrapper between 0.13 and 0.16).

Locally verified:
- both archs cross-compile green in docker
- resulting x86_64 binary (3.3 MB) runs in an alpine:latest container,
  --version / --help work, no dynamic lib requirements

The musl archive skips the UI (routers are headless) and swaps run.sh
for a procd init script (assets/openwrt/mhrv-rs.init) expecting the
binary at /usr/bin/mhrv-rs and config at /etc/mhrv-rs/config.json.

Side effect: switched tokio-rustls to default-features=false + ring
(was pulling aws-lc-rs transitively, which can't easily cross-compile
for musl). The main crate already uses ring explicitly, so no runtime
behavior change.

README gets a 'Running on OpenWRT (or any musl distro)' section in
both English and Persian with scp + procd enable/start recipe.
Closes #2.

assets/openwrt/mhrv-rs.init

@@ -0,0 +1,33 @@
+#!/bin/sh /etc/rc.common
+# OpenWRT procd init script for mhrv-rs.
+# Install as /etc/init.d/mhrv-rs, then:
+#   /etc/init.d/mhrv-rs enable
+#   /etc/init.d/mhrv-rs start
+#
+# Expects:
+#   /usr/bin/mhrv-rs           (the static musl binary from the release)
+#   /etc/mhrv-rs/config.json   (your config)
+
+START=99
+USE_PROCD=1
+
+BIN=/usr/bin/mhrv-rs
+CONFIG=/etc/mhrv-rs/config.json
+
+start_service() {
+    [ -x "$BIN" ] || return 1
+    [ -f "$CONFIG" ] || return 1
+
+    procd_open_instance
+    procd_set_param command "$BIN" --config "$CONFIG" --no-cert-check
+    procd_set_param respawn 3600 5 5
+    procd_set_param stdout 1
+    procd_set_param stderr 1
+    procd_set_param file "$CONFIG"
+    procd_close_instance
+}
+
+reload_service() {
+    stop
+    start
+}